diff --git a/Contains.go b/Contains.go
new file mode 100644
index 0000000..cc66a96
--- /dev/null
+++ b/Contains.go
@@ -0,0 +1,95 @@
+package assert
+
+import (
+	"reflect"
+	"strings"
+	"testing"
+)
+
+// Contains asserts that a contains b.
+func Contains(t testing.TB, a any, b any) {
+	if contains(a, b) {
+		return
+	}
+
+	t.Errorf(`
+file:     %s
+assert:   Contains
+container:%v
+element:  %v`, file(t), a, b)
+	t.FailNow()
+}
+
+// NotContains asserts that a doesn't contain b.
+func NotContains(t testing.TB, a any, b any) {
+	if !contains(a, b) {
+		return
+	}
+
+	t.Errorf(`
+file:     %s
+assert:   NotContains
+container:%v
+element:  %v`, file(t), a, b)
+	t.FailNow()
+}
+
+// contains returns whether container contains the given the element.
+// It works with strings, maps and slices.
+func contains(container any, element any) bool {
+	containerValue := reflect.ValueOf(container)
+
+	switch containerValue.Kind() {
+	case reflect.String:
+		elementValue := reflect.ValueOf(element)
+		return strings.Contains(containerValue.String(), elementValue.String())
+
+	case reflect.Map:
+		keys := containerValue.MapKeys()
+
+		for _, key := range keys {
+			if key.Interface() == element {
+				return true
+			}
+		}
+
+	case reflect.Slice:
+		elementValue := reflect.ValueOf(element)
+
+		if elementValue.Kind() == reflect.Slice {
+			elementLength := elementValue.Len()
+
+			if elementLength == 0 {
+				return true
+			}
+
+			if elementLength > containerValue.Len() {
+				return false
+			}
+
+			matchingElements := 0
+
+			for i := 0; i < containerValue.Len(); i++ {
+				if containerValue.Index(i).Interface() == elementValue.Index(matchingElements).Interface() {
+					matchingElements++
+				} else {
+					matchingElements = 0
+				}
+
+				if matchingElements == elementLength {
+					return true
+				}
+			}
+
+			return false
+		}
+
+		for i := 0; i < containerValue.Len(); i++ {
+			if containerValue.Index(i).Interface() == element {
+				return true
+			}
+		}
+	}
+
+	return false
+}
diff --git a/Contains_test.go b/Contains_test.go
new file mode 100644
index 0000000..c801692
--- /dev/null
+++ b/Contains_test.go
@@ -0,0 +1,29 @@
+package assert_test
+
+import (
+	"testing"
+
+	"git.akyoto.dev/go/assert"
+)
+
+func TestContains(t *testing.T) {
+	assert.Contains(t, "Hello", "H")
+	assert.Contains(t, "Hello", "Hello")
+	assert.Contains(t, []string{"Hello", "World"}, "Hello")
+	assert.Contains(t, []int{1, 2, 3}, 2)
+	assert.Contains(t, []int{1, 2, 3}, []int{})
+	assert.Contains(t, []int{1, 2, 3}, []int{1, 2})
+	assert.Contains(t, []byte{'H', 'e', 'l', 'l', 'o'}, byte('e'))
+	assert.Contains(t, []byte{'H', 'e', 'l', 'l', 'o'}, []byte{'e', 'l'})
+	assert.Contains(t, map[string]int{"Hello": 1, "World": 2}, "Hello")
+}
+
+func TestNotContains(t *testing.T) {
+	assert.NotContains(t, "Hello", "404")
+	assert.NotContains(t, []string{"Hello", "World"}, "404")
+	assert.NotContains(t, []int{1, 2, 3}, 4)
+	assert.NotContains(t, []int{1, 2, 3}, []int{2, 1})
+	assert.NotContains(t, []byte{'H', 'e', 'l', 'l', 'o'}, byte('a'))
+	assert.NotContains(t, []byte{'H', 'e', 'l', 'l', 'o'}, []byte{'l', 'e'})
+	assert.NotContains(t, map[string]int{"Hello": 1, "World": 2}, "404")
+}
diff --git a/Nil.go b/Nil.go
new file mode 100644
index 0000000..cc2aa9f
--- /dev/null
+++ b/Nil.go
@@ -0,0 +1,50 @@
+package assert
+
+import (
+	"reflect"
+	"testing"
+)
+
+// Nil asserts that the given parameter equals nil.
+func Nil(t testing.TB, a any) {
+	if isNil(a) {
+		return
+	}
+
+	t.Errorf(`
+file:     %s
+assert:   Nil
+value:    %v
+expected: nil`, file(t), a)
+	t.FailNow()
+}
+
+// NotNil asserts that the given parameter does not equal nil.
+func NotNil(t testing.TB, a any) {
+	if !isNil(a) {
+		return
+	}
+
+	t.Errorf(`
+file:     %s
+assert:   NotNil
+value:    %v
+expected: not nil`, file(t), a)
+	t.FailNow()
+}
+
+// isNil returns true if the object is nil.
+func isNil(object any) bool {
+	if object == nil {
+		return true
+	}
+
+	value := reflect.ValueOf(object)
+
+	switch value.Kind() {
+	case reflect.Chan, reflect.Func, reflect.Map, reflect.Ptr, reflect.UnsafePointer, reflect.Interface, reflect.Slice:
+		return value.IsNil()
+	}
+
+	return false
+}
diff --git a/Nil_test.go b/Nil_test.go
new file mode 100644
index 0000000..83ee265
--- /dev/null
+++ b/Nil_test.go
@@ -0,0 +1,25 @@
+
+package assert_test
+
+import (
+	"testing"
+
+	"git.akyoto.dev/go/assert"
+)
+
+func TestNil(t *testing.T) {
+	var nilPointer *T
+	var nilSlice []byte
+
+	assert.Nil(t, nil)
+	assert.Nil(t, nilPointer)
+	assert.Nil(t, nilSlice)
+}
+
+func TestNotNil(t *testing.T) {
+	assert.NotNil(t, 0)
+	assert.NotNil(t, "Hello")
+	assert.NotNil(t, []byte{})
+	assert.NotNil(t, T{})
+	assert.NotNil(t, &T{})
+}
diff --git a/True.go b/True.go
new file mode 100644
index 0000000..2e4b00b
--- /dev/null
+++ b/True.go
@@ -0,0 +1,13 @@
+package assert
+
+import "testing"
+
+// True asserts that the given parameter is true.
+func True(t testing.TB, a bool) {
+	Equal(t, a, true)
+}
+
+// False asserts that the given parameter is false.
+func False(t testing.TB, a bool) {
+	Equal(t, a, false)
+}
diff --git a/True_test.go b/True_test.go
new file mode 100644
index 0000000..2862862
--- /dev/null
+++ b/True_test.go
@@ -0,0 +1,12 @@
+package assert_test
+
+import (
+	"testing"
+
+	"git.akyoto.dev/go/assert"
+)
+
+func TestTrue(t *testing.T) {
+	assert.True(t, true)
+	assert.False(t, false)
+}