diff --git a/.gitignore b/.gitignore
index 45e2eb4..1e24dcf 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,3 +1,4 @@
 /target
 tmp/
 *.bundle
+.rspec_status
diff --git a/expression-ruby/ext/lago_expression/src/lib.rs b/expression-ruby/ext/lago_expression/src/lib.rs
index 8669b93..a614cf0 100644
--- a/expression-ruby/ext/lago_expression/src/lib.rs
+++ b/expression-ruby/ext/lago_expression/src/lib.rs
@@ -3,10 +3,10 @@ use std::collections::HashMap;
 use expression_core::{Event, Expression, ExpressionParser, ExpressionValue};
 use magnus::{function, method, value::ReprValue, Error, IntoValue, Module, Object, Ruby};
 
-#[magnus::wrap(class = "Expression", free_immediately, size)]
+#[magnus::wrap(class = "Lago::Expression", free_immediately, size)]
 struct ExpressionWrapper(Expression);
 
-#[magnus::wrap(class = "Event", free_immediately, size)]
+#[magnus::wrap(class = "Lago::Event", free_immediately, size)]
 struct EventWrapper(Event);
 
 impl EventWrapper {
diff --git a/expression-ruby/lib/lago-expression.rb b/expression-ruby/lib/lago-expression.rb
index 39d2f8d..e3374b8 100644
--- a/expression-ruby/lib/lago-expression.rb
+++ b/expression-ruby/lib/lago-expression.rb
@@ -1,3 +1,8 @@
 require 'bigdecimal/util'
 
 require_relative 'lago_expression/lago_expression'
+
+
+module Lago
+  VERSION = '0.0.1'.freeze
+end
diff --git a/expression-ruby/spec/lago-expression/expression_parser_spec.rb b/expression-ruby/spec/lago-expression/expression_parser_spec.rb
new file mode 100644
index 0000000..6f40daa
--- /dev/null
+++ b/expression-ruby/spec/lago-expression/expression_parser_spec.rb
@@ -0,0 +1,14 @@
+require 'spec_helper'
+
+
+RSpec.describe Lago::ExpressionParser do
+  describe '.parse' do
+    it "returns an expression when it's valid" do
+      expect(described_class.parse("1+2")).not_to be_nil
+    end
+
+    it "returns nil when it's not valid" do
+      expect(described_class.parse("1+")).to be_nil
+    end
+  end
+end
diff --git a/expression-ruby/spec/lago-expression/expression_spec.rb b/expression-ruby/spec/lago-expression/expression_spec.rb
new file mode 100644
index 0000000..40b539b
--- /dev/null
+++ b/expression-ruby/spec/lago-expression/expression_spec.rb
@@ -0,0 +1,50 @@
+require 'spec_helper'
+
+
+RSpec.describe Lago::Expression do
+  let(:event) { Lago::Event.new("code", 1234, {"property_1" => "1.23", "property_2" => "test", "property_3" => "12.34"}) }
+
+  describe '#evaluate' do
+    context "with a simple math expression" do
+      let(:expression) { Lago::ExpressionParser.parse("1 + 3") }
+
+      it "returns a bigdecimal" do
+        expect(expression.evaluate(event)).to eq(4.to_d)
+        expect(expression.evaluate(event)).to be_a(BigDecimal)
+      end
+    end
+
+    context "with a simple string expression" do
+      let(:expression) { Lago::ExpressionParser.parse("'test'") }
+
+      it "returns a string" do
+        expect(expression.evaluate(event)).to eq('test')
+        expect(expression.evaluate(event)).to be_a(String)
+      end
+    end
+
+    context "with a math expression with a decimal value from the event" do
+      let(:expression) { Lago::ExpressionParser.parse("(123 - event.properties.property_1) / 10") }
+
+      it "returns the calculated value" do
+        expect(expression.evaluate(event)).to eq(12.177)
+      end
+    end
+
+    context "with a concat function" do
+      let(:expression) { Lago::ExpressionParser.parse("concat(event.properties.property_2, '-', 'suffix')") }
+
+      it "concats the string" do
+        expect(expression.evaluate(event)).to eq('test-suffix')
+      end
+    end
+
+    context "with rounding function" do
+      let(:expression) { Lago::ExpressionParser.parse("round(event.properties.property_3, -1)") }
+
+      it "rounds the property" do
+        expect(expression.evaluate(event)).to eq(10)
+      end
+    end
+  end
+end
diff --git a/expression-ruby/spec/lago-expression/version_spec.rb b/expression-ruby/spec/lago-expression/version_spec.rb
new file mode 100644
index 0000000..56e154d
--- /dev/null
+++ b/expression-ruby/spec/lago-expression/version_spec.rb
@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+#
+require 'spec_helper'
+
+RSpec.describe Lago do
+  it 'has a version number' do
+    expect(Lago::VERSION).not_to be nil
+  end
+end
diff --git a/expression-ruby/spec/spec_helper.rb b/expression-ruby/spec/spec_helper.rb
new file mode 100644
index 0000000..1e6dacd
--- /dev/null
+++ b/expression-ruby/spec/spec_helper.rb
@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+require 'lago-expression'
+
+RSpec.configure do |config|
+
+  # Enable flags like --only-failures and --next-failure
+  config.example_status_persistence_file_path = '.rspec_status'
+
+  # Disable RSpec exposing methods globally on `Module` and `main`
+  config.disable_monkey_patching!
+
+  config.expect_with :rspec do |c|
+    c.syntax = :expect
+  end
+end