diff --git a/lib/fiken.rb b/lib/fiken.rb
new file mode 100644
index 0000000..efc05cc
--- /dev/null
+++ b/lib/fiken.rb
@@ -0,0 +1,47 @@
+# frozen_string_literal: true
+
+require_relative "fiken/version"
+require_relative "fiken/error"
+require_relative "fiken/object"
+require_relative "fiken/connection"
+require_relative "fiken/collection"
+require_relative "fiken/resource"
+
+# Shared sub-resources used by the resource definitions below.
+require_relative "fiken/resources/concerns/attachments"
+require_relative "fiken/resources/concerns/payments"
+require_relative "fiken/resources/concerns/drafts"
+
+# Resource definitions (one file per API tag).
+require_relative "fiken/resources/companies"
+require_relative "fiken/resources/accounts"
+require_relative "fiken/resources/bank_accounts"
+require_relative "fiken/resources/contacts"
+require_relative "fiken/resources/groups"
+require_relative "fiken/resources/products"
+require_relative "fiken/resources/journal_entries"
+require_relative "fiken/resources/transactions"
+require_relative "fiken/resources/invoices"
+require_relative "fiken/resources/credit_notes"
+require_relative "fiken/resources/offers"
+require_relative "fiken/resources/order_confirmations"
+require_relative "fiken/resources/sales"
+require_relative "fiken/resources/purchases"
+require_relative "fiken/resources/inbox"
+require_relative "fiken/resources/projects"
+require_relative "fiken/resources/activities"
+require_relative "fiken/resources/time_entries"
+require_relative "fiken/resources/time_users"
+
+require_relative "fiken/oauth"
+require_relative "fiken/client"
+
+# Ruby client for the Fiken API v2 (https://api.fiken.no/api/v2/docs).
+module Fiken
+  class << self
+    # Convenience constructor: Fiken.client(token: "...")
+    def client(**options)
+      Client.new(**options)
+    end
+  end
+end
diff --git a/lib/fiken/client.rb b/lib/fiken/client.rb
new file mode 100644
index 0000000..9672a8f
--- /dev/null
+++ b/lib/fiken/client.rb
@@ -0,0 +1,58 @@
+# frozen_string_literal: true
+
+module Fiken
+  # The entry point. Holds the connection and exposes the resource accessors.
+  #
+  #   client = Fiken::Client.new(token: ENV["FIKEN_TOKEN"])
+  #   client.user
+  #   client.companies.list
+  #   client.contacts("my-company-slug").find(123)
+  #   client.invoices("my-company-slug").drafts.create(invoice_attrs)
+  class Client
+    attr_reader :connection
+
+    def initialize(token: nil, access_token: nil, **options)
+      bearer = token || access_token
+      raise ArgumentError, "provide :token or :access_token" if bearer.nil? || bearer.empty?
+
+      @connection = Connection.new(token: bearer, **options)
+    end
+
+    # GET /user
+    def user
+      Object.new(connection.get("/user").body)
+    end
+
+    # GET /companies, GET /companies/{slug}
+    def companies
+      Resources::Companies.new(self)
+    end
+
+    # Convenience: fetch a single company by slug.
+    def company(slug)
+      companies.find(slug)
+    end
+
+    # Company-scoped resources. Each takes the company slug.
+    def accounts(slug) = Resources::Accounts.new(self, slug)
+    def account_balances(slug) = Resources::AccountBalances.new(self, slug)
+    def activities(slug) = Resources::Activities.new(self, slug)
+    def bank_accounts(slug) = Resources::BankAccounts.new(self, slug)
+    def bank_balances(slug) = Resources::BankBalances.new(self, slug)
+    def contacts(slug) = Resources::Contacts.new(self, slug)
+    def credit_notes(slug) = Resources::CreditNotes.new(self, slug)
+    def groups(slug) = Resources::Groups.new(self, slug)
+    def inbox(slug) = Resources::Inbox.new(self, slug)
+    def invoices(slug) = Resources::Invoices.new(self, slug)
+    def journal_entries(slug) = Resources::JournalEntries.new(self, slug)
+    def offers(slug) = Resources::Offers.new(self, slug)
+    def order_confirmations(slug) = Resources::OrderConfirmations.new(self, slug)
+    def products(slug) = Resources::Products.new(self, slug)
+    def projects(slug) = Resources::Projects.new(self, slug)
+    def purchases(slug) = Resources::Purchases.new(self, slug)
+    def sales(slug) = Resources::Sales.new(self, slug)
+    def time_entries(slug) = Resources::TimeEntries.new(self, slug)
+    def time_users(slug) = Resources::TimeUsers.new(self, slug)
+    def transactions(slug) = Resources::Transactions.new(self, slug)
+  end
+end
diff --git a/lib/fiken/resources/accounts.rb b/lib/fiken/resources/accounts.rb
new file mode 100644
index 0000000..9ae8409
--- /dev/null
+++ b/lib/fiken/resources/accounts.rb
@@ -0,0 +1,25 @@
+# frozen_string_literal: true
+
+module Fiken
+  module Resources
+    # /companies/{slug}/accounts — find by account code (e.g. "1500:10001").
+    class Accounts < Resource::Base
+      include Resource::Listable
+      include Resource::Findable
+
+      def resource_path
+        "accounts"
+      end
+    end
+
+    # /companies/{slug}/accountBalances
+    class AccountBalances < Resource::Base
+      include Resource::Listable
+      include Resource::Findable
+
+      def resource_path
+        "accountBalances"
+      end
+    end
+  end
+end
diff --git a/lib/fiken/resources/activities.rb b/lib/fiken/resources/activities.rb
new file mode 100644
index 0000000..c028eea
--- /dev/null
+++ b/lib/fiken/resources/activities.rb
@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+module Fiken
+  module Resources
+    # /companies/{slug}/activities (time-tracking activities)
+    class Activities < Resource::Base
+      include Resource::Listable
+      include Resource::Findable
+      include Resource::Creatable
+      include Resource::PatchUpdatable
+      include Resource::Deletable
+
+      def resource_path
+        "activities"
+      end
+    end
+  end
+end
diff --git a/lib/fiken/resources/bank_accounts.rb b/lib/fiken/resources/bank_accounts.rb
new file mode 100644
index 0000000..9fac46f
--- /dev/null
+++ b/lib/fiken/resources/bank_accounts.rb
@@ -0,0 +1,25 @@
+# frozen_string_literal: true
+
+module Fiken
+  module Resources
+    # /companies/{slug}/bankAccounts
+    class BankAccounts < Resource::Base
+      include Resource::Listable
+      include Resource::Findable
+      include Resource::Creatable
+
+      def resource_path
+        "bankAccounts"
+      end
+    end
+
+    # /companies/{slug}/bankBalances
+    class BankBalances < Resource::Base
+      include Resource::Listable
+
+      def resource_path
+        "bankBalances"
+      end
+    end
+  end
+end
diff --git a/lib/fiken/resources/companies.rb b/lib/fiken/resources/companies.rb
new file mode 100644
index 0000000..18efcf1
--- /dev/null
+++ b/lib/fiken/resources/companies.rb
@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+module Fiken
+  module Resources
+    # GET /companies and GET /companies/{companySlug}
+    class Companies < Resource::Base
+      include Resource::Listable
+      include Resource::Findable
+
+      def resource_path
+        "companies"
+      end
+    end
+  end
+end
diff --git a/lib/fiken/resources/contacts.rb b/lib/fiken/resources/contacts.rb
new file mode 100644
index 0000000..d9de274
--- /dev/null
+++ b/lib/fiken/resources/contacts.rb
@@ -0,0 +1,46 @@
+# frozen_string_literal: true
+
+module Fiken
+  module Resources
+    # /companies/{slug}/contacts and nested contact persons.
+    class Contacts < Resource::Base
+      include Resource::Listable
+      include Resource::Findable
+      include Resource::Creatable
+      include Resource::Updatable # PUT
+      include Resource::Deletable
+      include Resource::Attachable
+
+      def resource_path
+        "contacts"
+      end
+
+      # Contacts only support uploading attachments, not listing them.
+      def attachments_listable?
+        false
+      end
+
+      def contact_persons(contact_id)
+        ContactPersons.new(client, company_slug, "#{base_path}/#{contact_id}")
+      end
+    end
+
+    # /companies/{slug}/contacts/{id}/contactPerson
+    class ContactPersons < Resource::Base
+      include Resource::Listable
+      include Resource::Findable
+      include Resource::Creatable
+      include Resource::Updatable # PUT
+      include Resource::Deletable
+
+      def initialize(client, company_slug, parent_path)
+        super(client, company_slug)
+        @parent_path = parent_path
+      end
+
+      def base_path
+        "#{@parent_path}/contactPerson"
+      end
+    end
+  end
+end
diff --git a/lib/fiken/resources/credit_notes.rb b/lib/fiken/resources/credit_notes.rb
new file mode 100644
index 0000000..2317a3b
--- /dev/null
+++ b/lib/fiken/resources/credit_notes.rb
@@ -0,0 +1,32 @@
+# frozen_string_literal: true
+
+module Fiken
+  module Resources
+    # /companies/{slug}/creditNotes
+    class CreditNotes < Resource::Base
+      include Resource::Listable
+      include Resource::Findable
+      include Resource::Sendable
+      include Resource::HasCounter
+      include Resource::Draftable
+
+      def resource_path
+        "creditNotes"
+      end
+
+      def draft_create_action
+        "createCreditNote"
+      end
+
+      # POST /creditNotes/full — credit a whole invoice.
+      def create_full(attributes)
+        post_create("#{base_path}/full", attributes)
+      end
+
+      # POST /creditNotes/partial — credit selected lines/amounts.
+      def create_partial(attributes)
+        post_create("#{base_path}/partial", attributes)
+      end
+    end
+  end
+end
diff --git a/lib/fiken/resources/groups.rb b/lib/fiken/resources/groups.rb
new file mode 100644
index 0000000..8c290e7
--- /dev/null
+++ b/lib/fiken/resources/groups.rb
@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+module Fiken
+  module Resources
+    # /companies/{slug}/groups (contact groups)
+    class Groups < Resource::Base
+      include Resource::Listable
+
+      def resource_path
+        "groups"
+      end
+    end
+  end
+end
diff --git a/lib/fiken/resources/inbox.rb b/lib/fiken/resources/inbox.rb
new file mode 100644
index 0000000..6f2da1b
--- /dev/null
+++ b/lib/fiken/resources/inbox.rb
@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+module Fiken
+  module Resources
+    # /companies/{slug}/inbox — documents awaiting bookkeeping.
+    class Inbox < Resource::Base
+      include Resource::Listable
+      include Resource::Findable
+      include Resource::Deletable
+
+      def resource_path
+        "inbox"
+      end
+
+      # POST /inbox — multipart upload of an inbox document.
+      def create(path: nil, io: nil, filename: nil, content_type: "application/octet-stream", **fields)
+        parts = { "file" => build_file_part(path, io, filename, content_type) }
+        fields.each { |key, value| parts[key.to_s] = value.to_s unless value.nil? }
+        build_created(connection.post_multipart(base_path, parts))
+      end
+    end
+  end
+end
diff --git a/lib/fiken/resources/invoices.rb b/lib/fiken/resources/invoices.rb
new file mode 100644
index 0000000..198972d
--- /dev/null
+++ b/lib/fiken/resources/invoices.rb
@@ -0,0 +1,25 @@
+# frozen_string_literal: true
+
+module Fiken
+  module Resources
+    # /companies/{slug}/invoices
+    class Invoices < Resource::Base
+      include Resource::Listable
+      include Resource::Findable
+      include Resource::Creatable
+      include Resource::PatchUpdatable # finalized invoices update via PATCH
+      include Resource::Attachable
+      include Resource::Sendable
+      include Resource::HasCounter
+      include Resource::Draftable
+
+      def resource_path
+        "invoices"
+      end
+
+      def draft_create_action
+        "createInvoice"
+      end
+    end
+  end
+end
diff --git a/lib/fiken/resources/journal_entries.rb b/lib/fiken/resources/journal_entries.rb
new file mode 100644
index 0000000..616019d
--- /dev/null
+++ b/lib/fiken/resources/journal_entries.rb
@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+module Fiken
+  module Resources
+    # /companies/{slug}/journalEntries
+    class JournalEntries < Resource::Base
+      include Resource::Listable
+      include Resource::Findable
+      include Resource::Attachable
+
+      def resource_path
+        "journalEntries"
+      end
+
+      # Creation posts to a sibling path: POST /generalJournalEntries
+      def create_general(attributes)
+        post_create("/companies/#{company_slug}/generalJournalEntries", attributes)
+      end
+    end
+  end
+end
diff --git a/lib/fiken/resources/offers.rb b/lib/fiken/resources/offers.rb
new file mode 100644
index 0000000..1d67b27
--- /dev/null
+++ b/lib/fiken/resources/offers.rb
@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+module Fiken
+  module Resources
+    # /companies/{slug}/offers
+    class Offers < Resource::Base
+      include Resource::Listable
+      include Resource::Findable
+      include Resource::Sendable
+      include Resource::HasCounter
+      include Resource::Draftable
+
+      def resource_path
+        "offers"
+      end
+
+      def draft_create_action
+        "createOffer"
+      end
+    end
+  end
+end
diff --git a/lib/fiken/resources/order_confirmations.rb b/lib/fiken/resources/order_confirmations.rb
new file mode 100644
index 0000000..f8ec49b
--- /dev/null
+++ b/lib/fiken/resources/order_confirmations.rb
@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+
+module Fiken
+  module Resources
+    # /companies/{slug}/orderConfirmations
+    class OrderConfirmations < Resource::Base
+      include Resource::Listable
+      include Resource::Findable
+      include Resource::HasCounter
+      include Resource::Draftable
+
+      def resource_path
+        "orderConfirmations"
+      end
+
+      def draft_create_action
+        "createOrderConfirmation"
+      end
+
+      # POST /{id}/createInvoiceDraft — turn a confirmation into an invoice draft.
+      def create_invoice_draft(confirmation_id)
+        post_create("#{base_path}/#{confirmation_id}/createInvoiceDraft", nil)
+      end
+    end
+  end
+end
diff --git a/lib/fiken/resources/products.rb b/lib/fiken/resources/products.rb
new file mode 100644
index 0000000..ac15cae
--- /dev/null
+++ b/lib/fiken/resources/products.rb
@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+module Fiken
+  module Resources
+    # /companies/{slug}/products
+    class Products < Resource::Base
+      include Resource::Listable
+      include Resource::Findable
+      include Resource::Creatable
+      include Resource::Updatable # PUT
+      include Resource::Deletable
+
+      def resource_path
+        "products"
+      end
+
+      # POST /products/salesReport — returns an array of per-product report rows.
+      def sales_report(attributes)
+        body = connection.post("#{base_path}/salesReport", attributes).body
+        Array(body).map { |row| wrap(row) }
+      end
+    end
+  end
+end
diff --git a/lib/fiken/resources/projects.rb b/lib/fiken/resources/projects.rb
new file mode 100644
index 0000000..adb84a4
--- /dev/null
+++ b/lib/fiken/resources/projects.rb
@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+module Fiken
+  module Resources
+    # /companies/{slug}/projects
+    class Projects < Resource::Base
+      include Resource::Listable
+      include Resource::Findable
+      include Resource::Creatable
+      include Resource::PatchUpdatable
+      include Resource::Deletable
+
+      def resource_path
+        "projects"
+      end
+    end
+  end
+end
diff --git a/lib/fiken/resources/purchases.rb b/lib/fiken/resources/purchases.rb
new file mode 100644
index 0000000..2d3ad23
--- /dev/null
+++ b/lib/fiken/resources/purchases.rb
@@ -0,0 +1,29 @@
+# frozen_string_literal: true
+
+module Fiken
+  module Resources
+    # /companies/{slug}/purchases
+    class Purchases < Resource::Base
+      include Resource::Listable
+      include Resource::Findable
+      include Resource::Creatable
+      include Resource::Attachable
+      include Resource::Payable
+      include Resource::Draftable
+
+      def resource_path
+        "purchases"
+      end
+
+      def draft_create_action
+        "createPurchase"
+      end
+
+      # PATCH /{id}/delete — delete a purchase (with a reason).
+      def delete(id, attributes = nil)
+        connection.patch("#{base_path}/#{id}/delete", attributes)
+        true
+      end
+    end
+  end
+end
diff --git a/lib/fiken/resources/sales.rb b/lib/fiken/resources/sales.rb
new file mode 100644
index 0000000..eb2b662
--- /dev/null
+++ b/lib/fiken/resources/sales.rb
@@ -0,0 +1,39 @@
+# frozen_string_literal: true
+
+module Fiken
+  module Resources
+    # /companies/{slug}/sales
+    class Sales < Resource::Base
+      include Resource::Listable
+      include Resource::Findable
+      include Resource::Creatable
+      include Resource::Attachable
+      include Resource::Payable
+      include Resource::Draftable
+
+      def resource_path
+        "sales"
+      end
+
+      def draft_create_action
+        "createSale"
+      end
+
+      # PATCH /{id}/settled — mark a sale as settled.
+      def settle(id, attributes = nil)
+        patch_one("#{base_path}/#{id}/settled", attributes)
+      end
+
+      # PATCH /{id}/writeOff — write off a sale as a loss.
+      def write_off(id, attributes = nil)
+        patch_one("#{base_path}/#{id}/writeOff", attributes)
+      end
+
+      # PATCH /{id}/delete — delete a sale (with a reason).
+      def delete(id, attributes = nil)
+        connection.patch("#{base_path}/#{id}/delete", attributes)
+        true
+      end
+    end
+  end
+end
diff --git a/lib/fiken/resources/time_entries.rb b/lib/fiken/resources/time_entries.rb
new file mode 100644
index 0000000..59d0f2e
--- /dev/null
+++ b/lib/fiken/resources/time_entries.rb
@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+module Fiken
+  module Resources
+    # /companies/{slug}/timeEntries
+    class TimeEntries < Resource::Base
+      include Resource::Listable
+      include Resource::Findable
+      include Resource::Creatable
+      include Resource::PatchUpdatable
+      include Resource::Deletable
+
+      def resource_path
+        "timeEntries"
+      end
+
+      # POST /timeEntries/createInvoiceDraft — invoice selected time entries.
+      def create_invoice_draft(attributes)
+        post_create("#{base_path}/createInvoiceDraft", attributes)
+      end
+    end
+  end
+end
diff --git a/lib/fiken/resources/time_users.rb b/lib/fiken/resources/time_users.rb
new file mode 100644
index 0000000..c5cd36f
--- /dev/null
+++ b/lib/fiken/resources/time_users.rb
@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+module Fiken
+  module Resources
+    # /companies/{slug}/timeUsers
+    class TimeUsers < Resource::Base
+      include Resource::Listable
+      include Resource::Findable
+
+      def resource_path
+        "timeUsers"
+      end
+    end
+  end
+end
diff --git a/lib/fiken/resources/transactions.rb b/lib/fiken/resources/transactions.rb
new file mode 100644
index 0000000..3bf9a90
--- /dev/null
+++ b/lib/fiken/resources/transactions.rb
@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+module Fiken
+  module Resources
+    # /companies/{slug}/transactions
+    class Transactions < Resource::Base
+      include Resource::Listable
+      include Resource::Findable
+
+      def resource_path
+        "transactions"
+      end
+
+      # Deletion is a PATCH to /{id}/delete (optionally with a description).
+      def delete(id, attributes = nil)
+        connection.patch("#{base_path}/#{id}/delete", attributes)
+        true
+      end
+    end
+  end
+end