diff --git a/.rubocop.yml b/.rubocop.yml
index 5c614cd..3932dfe 100644
--- a/.rubocop.yml
+++ b/.rubocop.yml
@@ -9,6 +9,14 @@ AllCops:
 Lint/MissingSuper:
   Enabled: false
 
+Metrics/MethodLength:
+  Enabled: true
+  Exclude:
+  - "lib/mars/workflows/parallel.rb"
+
+RSpec/ExampleLength:
+  Enabled: false
+
 Style/Documentation:
   Enabled: false
 
diff --git a/examples/parallel_workflow/diagram.md b/examples/parallel_workflow/diagram.md
index 24a4829..0efc8cb 100644
--- a/examples/parallel_workflow/diagram.md
+++ b/examples/parallel_workflow/diagram.md
@@ -2,15 +2,15 @@
 flowchart LR
 in((In))
 out((Out))
-parallel_workflow_aggregator[Parallel workflow Aggregator]
+aggregator[Aggregator]
 llm_1[LLM 1]
 llm_2[LLM 2]
 llm_3[LLM 3]
 in --> llm_1
 in --> llm_2
 in --> llm_3
-llm_1 --> parallel_workflow_aggregator
-parallel_workflow_aggregator --> out
-llm_2 --> parallel_workflow_aggregator
-llm_3 --> parallel_workflow_aggregator
+llm_1 --> aggregator
+aggregator --> out
+llm_2 --> aggregator
+llm_3 --> aggregator
 ```
diff --git a/examples/parallel_workflow/examples.md b/examples/parallel_workflow/examples.md
deleted file mode 100644
index 3399381..0000000
--- a/examples/parallel_workflow/examples.md
+++ /dev/null
@@ -1,26 +0,0 @@
-```mermaid
-flowchart LR
-In(("In")) -->
-LLM_1["LLM 1"] & LLM_2["LLM 2"] & LLM_3["LLM 3"]
-LLM_1 --> LLM_4["LLM_4"]
-LLM_4 --> AGGREGATOR
-LLM_2 --> AGGREGATOR
-LLM_3 --> AGGREGATOR
-AGGREGATOR --> Out(("Out"))
-```
-#
-
-
-```mermaid
-flowchart LR
-In(("In")) --> LLM_1["LLM 1"]
-In(("In")) --> LLM_2["LLM 2"]
-In(("In")) --> LLM_3["LLM 3"]
-
-LLM_1 --> LLM_4["LLM_4"]
-LLM_4 --> AGGREGATOR
-LLM_2 --> AGGREGATOR
-LLM_3 --> AGGREGATOR
-AGGREGATOR --> Out(("Out"))
-```
-#
diff --git a/examples/parallel_workflow/generator.rb b/examples/parallel_workflow/generator.rb
index 069b542..106a385 100755
--- a/examples/parallel_workflow/generator.rb
+++ b/examples/parallel_workflow/generator.rb
@@ -10,10 +10,13 @@
 
 llm3 = Mars::Agent.new(name: "LLM 3")
 
+aggregator = Mars::Aggregator.new("Aggregator", operation: lambda(&:sum))
+
 # Create the parallel workflow (LLM 1, LLM 2, LLM 3)
 parallel_workflow = Mars::Workflows::Parallel.new(
   "Parallel workflow",
-  steps: [llm1, llm2, llm3]
+  steps: [llm1, llm2, llm3],
+  aggregator: aggregator
 )
 
 # Generate and save the diagram
diff --git a/lib/mars.rb b/lib/mars.rb
index 831d67c..06cce5d 100644
--- a/lib/mars.rb
+++ b/lib/mars.rb
@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require "zeitwerk"
+require "async"
 
 loader = Zeitwerk::Loader.for_gem
 loader.setup
diff --git a/lib/mars/aggregator.rb b/lib/mars/aggregator.rb
index b660b76..a4a7225 100644
--- a/lib/mars/aggregator.rb
+++ b/lib/mars/aggregator.rb
@@ -2,16 +2,15 @@
 
 module Mars
   class Aggregator < Runnable
-    attr_reader :name
+    attr_reader :name, :operation
 
-    def initialize(name = "Aggregator")
+    def initialize(name = "Aggregator", operation: nil)
       @name = name
+      @operation = operation || ->(inputs) { inputs.join("\n") }
     end
 
     def run(inputs)
-      return yield if block_given?
-
-      inputs.join("\n")
+      operation.call(inputs)
     end
   end
 end
diff --git a/lib/mars/workflows/aggregate_error.rb b/lib/mars/workflows/aggregate_error.rb
new file mode 100644
index 0000000..092545e
--- /dev/null
+++ b/lib/mars/workflows/aggregate_error.rb
@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+module Mars
+  module Workflows
+    class AggregateError < StandardError
+      attr_reader :errors
+
+      def initialize(errors)
+        @errors = errors
+        super(errors.map { |error| "#{error[:step_name]}: #{error[:error].message}" }.join("\n"))
+      end
+    end
+  end
+end
diff --git a/lib/mars/workflows/parallel.rb b/lib/mars/workflows/parallel.rb
index 6332827..41b3c34 100644
--- a/lib/mars/workflows/parallel.rb
+++ b/lib/mars/workflows/parallel.rb
@@ -12,11 +12,22 @@ def initialize(name, steps:, aggregator: nil)
       end
 
       def run(input)
-        inputs = @steps.map do |step|
-          step.run(input)
-        end
+        errors = []
+        results = Async do |workflow|
+          tasks = @steps.map do |step|
+            workflow.async do
+              step.run(input)
+            rescue StandardError => e
+              errors << { error: e, step_name: step.name }
+            end
+          end
 
-        aggregator.run(inputs)
+          tasks.map(&:wait)
+        end.result
+
+        raise AggregateError, errors if errors.any?
+
+        aggregator.run(results)
       end
 
       private
diff --git a/mars.gemspec b/mars.gemspec
index 6317bac..3d4bd97 100644
--- a/mars.gemspec
+++ b/mars.gemspec
@@ -35,6 +35,7 @@ Gem::Specification.new do |spec|
   spec.require_paths = ["lib"]
 
   # Uncomment to register a new dependency of your gem
+  spec.add_dependency "async", "~> 2.34"
   spec.add_dependency "ruby_llm", "~> 1.0"
   spec.add_dependency "zeitwerk", "~> 2.7"
 
diff --git a/spec/mars/workflows/parallel_spec.rb b/spec/mars/workflows/parallel_spec.rb
new file mode 100644
index 0000000..37ac7be
--- /dev/null
+++ b/spec/mars/workflows/parallel_spec.rb
@@ -0,0 +1,103 @@
+# frozen_string_literal: true
+
+RSpec.describe Mars::Workflows::Parallel do
+  let(:add_step_class) do
+    Class.new do
+      def initialize(value)
+        @value = value
+      end
+
+      def run(input)
+        sleep 0.1
+        puts "add step: #{input}"
+        input + @value
+      end
+    end
+  end
+
+  let(:multiply_step_class) do
+    Class.new do
+      def initialize(multiplier)
+        @multiplier = multiplier
+      end
+
+      def run(input)
+        puts "multiply step: #{input}"
+        input * @multiplier
+      end
+    end
+  end
+
+  let(:error_step_class) do
+    Class.new do
+      attr_reader :name
+
+      def initialize(message, name)
+        @message = message
+        @name = name
+      end
+
+      def run(_input)
+        puts "error step: #{@name}"
+        raise StandardError, @message
+      end
+    end
+  end
+
+  describe "#run" do
+    it "executes steps in parallel" do
+      add_five = add_step_class.new(5)
+      multiply_three = multiply_step_class.new(3)
+      add_two = add_step_class.new(2)
+
+      workflow = described_class.new("math_workflow", steps: [add_five, multiply_three, add_two])
+
+      # 10 + 5 = 15, 10 * 3 = 30, 10 + 2 = 12
+      expect(workflow.run(10)).to eq("15\n30\n12")
+    end
+
+    it "executes steps in parallel with a custom aggregator" do
+      add_five = add_step_class.new(5)
+      multiply_three = multiply_step_class.new(3)
+      add_two = add_step_class.new(2)
+      aggregator = Mars::Aggregator.new("Custom Aggregator", operation: lambda(&:sum))
+      workflow = described_class.new("math_workflow", steps: [add_five, multiply_three, add_two],
+                                                      aggregator: aggregator)
+
+      expect(workflow.run(10)).to eq(57)
+    end
+
+    it "handles single step" do
+      multiply_step = multiply_step_class.new(7)
+      workflow = described_class.new("single_step", steps: [multiply_step])
+
+      expect(workflow.run(6)).to eq("42")
+    end
+
+    it "returns input unchanged when no steps" do
+      workflow = described_class.new("empty", steps: [])
+
+      expect(workflow.run(42)).to eq("")
+    end
+
+    it "propagates errors from steps" do
+      add_step = add_step_class.new(5)
+      error_step = error_step_class.new("Step failed", "error_step_one")
+      error_step_two = error_step_class.new("Step failed two", "error_step_two")
+
+      workflow = described_class.new("error_workflow", steps: [add_step, error_step, error_step_two])
+
+      expect { workflow.run(10) }.to raise_error(
+        Mars::Workflows::AggregateError,
+        "error_step_one: Step failed\nerror_step_two: Step failed two"
+      )
+    end
+  end
+
+  describe "inheritance" do
+    it "inherits from Mars::Runnable" do
+      workflow = described_class.new("test", steps: [])
+      expect(workflow).to be_a(Mars::Runnable)
+    end
+  end
+end