diff --git a/src/typed_expression/validate.rs b/src/typed_expression/validate.rs
index 36ae73a..4c88217 100644
--- a/src/typed_expression/validate.rs
+++ b/src/typed_expression/validate.rs
@@ -115,12 +115,36 @@ impl Expression {
 
             // Arithmetic operators
             Expression::Add { left, right } => {
-                validate_binary_arithmetic(left, right, df_type, "addition", |l, r, et| {
-                    TypedExpression::Add {
-                        left: Arc::new(l),
-                        right: Arc::new(r),
-                        expression_type: et,
-                    }
+                let typed_left = left.validate(df_type)?;
+                let typed_right = right.validate(df_type)?;
+                let left_type = typed_left.expression_type();
+                let right_type = typed_right.expression_type();
+                let left_dt = left_type.data_type();
+                let right_dt = right_type.data_type();
+
+                let is_addition = left_dt.is_numeric() && right_dt.is_numeric();
+                let is_concatenation = matches!(
+                    (left_dt, right_dt),
+                    (
+                        DataType::String | DataType::Nothing,
+                        DataType::String | DataType::Nothing
+                    )
+                );
+
+                if !is_addition && !is_concatenation {
+                    return Err(ValidationError::IncompatibleTypes {
+                        operation: "addition".to_string(),
+                        left_type: left_dt,
+                        right_type: right_dt,
+                    });
+                }
+
+                let result_type = promote_expression_types(left_type, right_type, "addition")?;
+                let result_dt = result_type.data_type();
+                Ok(TypedExpression::Add {
+                    left: Arc::new(typed_left.cast_if_needed(result_dt)),
+                    right: Arc::new(typed_right.cast_if_needed(result_dt)),
+                    expression_type: result_type,
                 })
             }
 
diff --git a/tests/operators/test_add.py b/tests/operators/test_add.py
index 63958f6..675bc02 100644
--- a/tests/operators/test_add.py
+++ b/tests/operators/test_add.py
@@ -3,6 +3,7 @@
 import pytest
 
 from tabeline import Array, DataFrame, DataType
+from tabeline.exceptions import IncompatibleTypesError
 from tabeline.testing import assert_data_frames_equal
 
 from .._types import (
@@ -123,3 +124,100 @@ def test_add_with_decimal_literal(original_dtype, expected_dtype, expression):
     actual = df.transmute(result=expression)
     expected = DataFrame(result=Array[expected_dtype](4.5, 6.5, None))
     assert_data_frames_equal(actual, expected, absolute_tolerance=absolute_tolerance)
+
+
+@pytest.mark.parametrize(
+    ("left", "right", "answer"),
+    [
+        ("a", "b", "ab"),
+        ("", "x", "x"),
+        ("x", "", "x"),
+        ("x", None, None),
+        (None, "x", None),
+        (None, None, None),
+    ],
+)
+def test_concatenate_strings(left, right, answer):
+    df = DataFrame(a=Array[DataType.String](left), b=Array[DataType.String](right))
+    actual = df.transmute(c="a + b")
+    expected = DataFrame(c=Array[DataType.String](answer))
+    assert_data_frames_equal(actual, expected)
+
+
+@pytest.mark.parametrize(
+    ("value", "answer"),
+    [
+        ("hello", "hello world"),
+        ("", " world"),
+        (None, None),
+    ],
+)
+def test_concatenate_column_with_literal(value, answer):
+    df = DataFrame(a=Array[DataType.String](value))
+    actual = df.transmute(c="a + ' world'")
+    expected = DataFrame(c=Array[DataType.String](answer))
+    assert_data_frames_equal(actual, expected)
+
+
+@pytest.mark.parametrize(
+    ("value", "answer"),
+    [
+        ("world", "hello world"),
+        ("", "hello "),
+        (None, None),
+    ],
+)
+def test_concatenate_literal_with_column(value, answer):
+    df = DataFrame(a=Array[DataType.String](value))
+    actual = df.transmute(c="'hello ' + a")
+    expected = DataFrame(c=Array[DataType.String](answer))
+    assert_data_frames_equal(actual, expected)
+
+
+@pytest.mark.parametrize(
+    ("string_value", "answer"),
+    [
+        ("x", None),
+        (None, None),
+    ],
+)
+def test_concatenate_string_with_nothing(string_value, answer):
+    df = DataFrame(a=Array[DataType.String](string_value), b=Array[DataType.Nothing](None))
+    actual = df.transmute(c="a + b")
+    expected = DataFrame(c=Array[DataType.Nothing](answer))
+    assert_data_frames_equal(actual, expected)
+
+
+@pytest.mark.parametrize(
+    ("string_value", "answer"),
+    [
+        ("x", None),
+        (None, None),
+    ],
+)
+def test_concatenate_nothing_with_string(string_value, answer):
+    df = DataFrame(a=Array[DataType.Nothing](None), b=Array[DataType.String](string_value))
+    actual = df.transmute(c="a + b")
+    expected = DataFrame(c=Array[DataType.Nothing](answer))
+    assert_data_frames_equal(actual, expected)
+
+
+@pytest.mark.parametrize(
+    ("left_values", "right_values", "left_type", "right_type"),
+    [
+        ([1, 2, 3], [True, False, True], DataType.Integer64, DataType.Boolean),
+        ([1, 2, 3], ["a", "b", "c"], DataType.Integer64, DataType.String),
+        (["a", "b", "c"], [1, 2, 3], DataType.String, DataType.Integer64),
+        (["a", "b", "c"], [True, False, True], DataType.String, DataType.Boolean),
+        ([True, False, True], [True, False, True], DataType.Boolean, DataType.Boolean),
+        ([True, False, True], [1, 2, 3], DataType.Boolean, DataType.Integer64),
+        ([True, False, True], ["a", "b", "c"], DataType.Boolean, DataType.String),
+    ],
+)
+def test_addition_rejects_incompatible_operand(left_values, right_values, left_type, right_type):
+    df = DataFrame(x=left_values, y=right_values)
+
+    with pytest.raises(IncompatibleTypesError) as exc_info:
+        df.mutate(z="x + y")
+
+    assert exc_info.value == IncompatibleTypesError("addition", left_type, right_type)
diff --git a/tests/operators/test_validation.py b/tests/operators/test_validation.py
index 09547ef..00ff4e7 100644
--- a/tests/operators/test_validation.py
+++ b/tests/operators/test_validation.py
@@ -7,7 +7,7 @@
 @pytest.mark.parametrize(
     ("expression", "operation"),
     [
-        ("x + y", "addition"),
+        # No `x + y` because `+` also does string concatenation
         ("x - y", "subtraction"),
         ("x * y", "multiplication"),
         ("x / y", "division"),