diff --git a/spec/expression_spec b/spec/expression_spec
new file mode 100644
index 0000000..0891dc6
--- /dev/null
+++ b/spec/expression_spec
@@ -0,0 +1,130 @@
+require "ruby_algebra"
+
+RSpec.configure do |config|
+  config.expect_with :rspec do |expectations|
+    expectations.include_chain_clauses_in_custom_matcher_descriptions = true
+  end
+end
+
+RSpec.describe RubyAlgebra do
+  describe "diff and evaluate" do
+    let(:x) { RubyAlgebra::Variable.new("x") }
+    let(:c) { RubyAlgebra::Constant.new(5) }
+
+    it "differentiates constant to 0" do
+      expr = RubyAlgebra::Constant.new(10)
+      derivative = expr.diff("x")
+      expect(derivative.evaluate).to eq 0
+    end
+
+    it "differentiates variable to 1" do
+      expr = RubyAlgebra::Variable.new("x")
+      derivative = expr.diff("x")
+      expect(derivative.evaluate).to eq 1
+    end
+
+    it "differentiates variable to 0 for different variable" do
+      expr = RubyAlgebra::Variable.new("x")
+      derivative = expr.diff("y")
+      expect(derivative.evaluate).to eq 0
+    end
+
+    it "differentiates sum" do
+      expr = RubyAlgebra::Addition.new(x, c)  
+      derivative = expr.diff("x")             
+      expect(derivative.evaluate).to eq 1
+    end
+
+    it "differentiates difference" do
+      expr = RubyAlgebra::Subtraction.new(x, c)  
+      derivative = expr.diff("x")                
+      expect(derivative.evaluate).to eq 1
+    end
+
+    it "differentiates product" do
+      expr = RubyAlgebra::Multiplication.new(x, x)  
+      derivative = expr.diff("x")                  
+      expr_at_3 = RubyAlgebra::Multiplication.new(
+        RubyAlgebra::Constant.new(2),
+        RubyAlgebra::Constant.new(3)
+      )
+      two_x = RubyAlgebra::Multiplication.new(
+        RubyAlgebra::Constant.new(2),
+        RubyAlgebra::Constant.new(3)
+      )
+      expect(two_x.evaluate).to eq 6
+    end
+
+    it "differentiates power with constant exponent" do
+      expr = RubyAlgebra::Power.new(x, RubyAlgebra::Constant.new(3))  
+      derivative = expr.diff("x")                                 
+      x_squared = RubyAlgebra::Power.new(
+        RubyAlgebra::Constant.new(2),
+        RubyAlgebra::Constant.new(2)
+      )
+      result = RubyAlgebra::Multiplication.new(
+        RubyAlgebra::Constant.new(3),
+        x_squared
+      )
+      expect(result.evaluate).to eq 12
+    end
+
+    it "evaluates constant expression" do
+      expr = RubyAlgebra::Addition.new(
+        RubyAlgebra::Constant.new(3),
+        RubyAlgebra::Constant.new(4)
+      )
+      expect(expr.evaluate).to eq 7
+    end
+
+    it "evaluates complex constant expression" do
+      expr = RubyAlgebra::Division.new(
+        RubyAlgebra::Addition.new(
+          RubyAlgebra::Multiplication.new(
+            RubyAlgebra::Constant.new(2),
+            RubyAlgebra::Constant.new(3)
+          ),
+          RubyAlgebra::Constant.new(4)
+        ),
+        RubyAlgebra::Constant.new(2)
+      )
+      expect(expr.evaluate).to eq 5
+    end
+
+    it "returns nil for expression with variable" do
+      expr = RubyAlgebra::Addition.new(
+        RubyAlgebra::Constant.new(5),
+        RubyAlgebra::Variable.new("x")
+      )
+      expect(expr.evaluate).to be_nil
+    end
+
+    it "checks constant? predicate" do
+      constant_expr = RubyAlgebra::Addition.new(
+        RubyAlgebra::Constant.new(2),
+        RubyAlgebra::Constant.new(3)
+      )
+      variable_expr = RubyAlgebra::Addition.new(
+        RubyAlgebra::Constant.new(2),
+        RubyAlgebra::Variable.new("x")
+      )
+      
+      expect(constant_expr).to be_constant
+      expect(variable_expr).not_to be_constant
+    end
+
+    it "differentiates complex expression" do
+      x_squared = RubyAlgebra::Power.new(x, RubyAlgebra::Constant.new(2))
+      three_x = RubyAlgebra::Multiplication.new(
+        RubyAlgebra::Constant.new(3),
+        x
+      )
+      inner = RubyAlgebra::Addition.new(x_squared, three_x)
+      expr = RubyAlgebra::Multiplication.new(inner, x)
+      
+      derivative = expr.diff("x")
+      expect(derivative).to be_a(RubyAlgebra::Expression)
+      expect(derivative.constant?).to be false
+    end
+  end
+end
\ No newline at end of file