diff --git a/README.md b/README.md
index 23731b0..cb97f8a 100644
--- a/README.md
+++ b/README.md
@@ -94,6 +94,24 @@ You can also give a default value for an argument.
 argument :name, default: "Benjamin"
 ```
 
+Defaults can also be callable (procs or lambdas) that are evaluated at runtime. Callable defaults have access to other attributes and are processed in definition order:
+
+```ruby
+class GreetingService < ApplicationService
+  argument :first_name, default: "John"
+  argument :last_name, default: "Doe"
+  argument :full_name, default: -> { "#{first_name} #{last_name}" }
+  argument :created_at, default: -> { Time.current }
+
+  def call
+    "Hello, #{full_name}!"
+  end
+end
+
+GreetingService.call.result #=> "Hello, John Doe!"
+GreetingService.call(first_name: "Jane").result #=> "Hello, Jane Doe!"
+```
+
 ### Validations
 
 Since `Telephone::Service` includes `ActiveModel::Model`, you can define validations in the same way you would for an ActiveRecord model.
diff --git a/lib/telephone/service.rb b/lib/telephone/service.rb
index 68cff4f..c8bb946 100644
--- a/lib/telephone/service.rb
+++ b/lib/telephone/service.rb
@@ -20,10 +20,24 @@ class Service
     # Primary responsibility of initialize is to instantiate the
     # attributes of the service object with the expected values.
     def initialize(attributes = {})
-      self.class.defaults.merge(attributes).each do |key, value|
+      attributes.each do |key, value|
         send("#{key}=", value)
       end
 
+      self.class.defaults.each do |key, value|
+        next if attributes.key?(key)
+
+        resolved = if value.is_a?(Proc)
+          instance_exec(&value)
+        elsif value.respond_to?(:call)
+          value.call
+        else
+          value
+        end
+
+        send("#{key}=", resolved)
+      end
+
       super
       yield self if block_given?
     end
@@ -51,14 +65,27 @@ class << self
       # to pass in a default, or set the argument to "required" to add a validation
       # that runs before executing the block.
       #
+      # The default value can be a static value or any callable object (Proc, lambda,
+      # method, or any object that responds to #call) that will be evaluated at
+      # runtime when the service is instantiated.
+      #
+      # Callable defaults are evaluated in the context of the service instance,
+      # so they can access other attributes. They are processed in definition order,
+      # meaning a callable can depend on any argument defined before it.
+      #
+      # To store a Proc as the actual value, wrap it in another lambda:
+      #   argument :my_proc, default: -> { -> { puts "hi" } }
+      #
       # @example
       #   class SomeService < Telephone::Service
-      #     argument :foo, default: "bar"
-      #     argument :baz, required: true
+      #     argument :first_name, default: "John"
+      #     argument :last_name, default: "Doe"
+      #     argument :full_name, default: -> { "#{first_name} #{last_name}" }
+      #     argument :timestamp, default: -> { DateTime.current }
       #
       #     def call
-      #       puts foo
-      #       puts baz
+      #       puts full_name
+      #       puts timestamp
       #     end
       #   end
       def argument(arg, default: nil, required: false)
diff --git a/spec/telephone_spec.rb b/spec/telephone_spec.rb
index 650e6c3..940d3e9 100644
--- a/spec/telephone_spec.rb
+++ b/spec/telephone_spec.rb
@@ -30,6 +30,117 @@ def call
 
       expect(subject.call.success?).to be false
     end
+
+    context "with callable defaults" do
+      it "evaluates callable defaults fresh on each instantiation" do
+        counter = {value: 0}
+        service = Class.new(Telephone::Service) do
+          argument :count, default: proc { counter[:value] += 1 }
+
+          def call
+            count
+          end
+        end
+
+        expect(service.new.count).to eq 1
+        expect(service.new.count).to eq 2
+        expect(service.new.count).to eq 3
+      end
+
+      it "works with any object responding to #call" do
+        callable_object = Class.new do
+          def call
+            "from callable object"
+          end
+        end.new
+
+        service = Class.new(Telephone::Service) do
+          argument :value, default: callable_object
+
+          def call
+            value
+          end
+        end
+
+        expect(service.new.value).to eq "from callable object"
+      end
+
+      it "allows overriding callable defaults with explicit values" do
+        service = Class.new(Telephone::Service) do
+          argument :value, default: -> { "default" }
+        end
+
+        expect(service.new(value: "custom").value).to eq "custom"
+      end
+
+      it "does not call non-callable defaults" do
+        service = Class.new(Telephone::Service) do
+          argument :data, default: "static string"
+
+          def call
+            data
+          end
+        end
+
+        expect(service.new.data).to eq "static string"
+      end
+
+      it "allows callables to access other attributes" do
+        service = Class.new(Telephone::Service) do
+          argument :first_name, default: "John"
+          argument :last_name, default: "Doe"
+          argument :full_name, default: -> { "#{first_name} #{last_name}" }
+
+          def call
+            full_name
+          end
+        end
+
+        expect(service.new.full_name).to eq "John Doe"
+      end
+
+      it "allows callables to depend on other callables in definition order" do
+        service = Class.new(Telephone::Service) do
+          argument :first_name, default: "John"
+          argument :last_name, default: "Doe"
+          argument :full_name, default: -> { "#{first_name} #{last_name}" }
+          argument :greeting, default: -> { "Hello, #{full_name}!" }
+
+          def call
+            greeting
+          end
+        end
+
+        expect(service.new.greeting).to eq "Hello, John Doe!"
+      end
+
+      it "allows callables to access explicitly provided attributes" do
+        service = Class.new(Telephone::Service) do
+          argument :first_name
+          argument :last_name
+          argument :full_name, default: -> { "#{first_name} #{last_name}" }
+
+          def call
+            full_name
+          end
+        end
+
+        expect(service.new(first_name: "Jane", last_name: "Smith").full_name).to eq "Jane Smith"
+      end
+
+      it "processes explicit attributes before callable defaults" do
+        service = Class.new(Telephone::Service) do
+          argument :name, default: "Default"
+          argument :message, default: -> { "Hello, #{name}!" }
+
+          def call
+            message
+          end
+        end
+
+        expect(service.new(name: "Custom").message).to eq "Hello, Custom!"
+      end
+    end
   end
 
   describe "#new" do