diff --git a/tests/Model/Integer/IntegerTest.php b/tests/Model/Integer/IntegerTest.php
new file mode 100644
index 0000000..948bb87
--- /dev/null
+++ b/tests/Model/Integer/IntegerTest.php
@@ -0,0 +1,112 @@
+<?php
+
+declare(strict_types=1);
+
+/*
+ * This file is part of OpenSolid package.
+ *
+ * (c) Yonel Ceruto <open@yceruto.dev>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace OpenSolid\Domain\Tests\Model\Integer;
+
+use OpenSolid\Domain\Model\Integer\Integer;
+use PHPUnit\Framework\TestCase;
+
+class IntegerTest extends TestCase
+{
+    public function testCreate(): void
+    {
+        $i = Integer::from(1);
+
+        $this->assertSame(1, $i->value());
+    }
+
+    public function testEquals(): void
+    {
+        $i = Integer::from(1);
+        $j = Integer::from(1);
+        $k = Integer::from(2);
+
+        $this->assertTrue($i->equals($j));
+        $this->assertFalse($i->equals($k));
+    }
+
+    public function testGreaterThan(): void
+    {
+        $i = Integer::from(2);
+        $j = Integer::from(1);
+        $k = Integer::from(2);
+
+        $this->assertTrue($i->greaterThan($j));
+        $this->assertFalse($i->greaterThan($k));
+    }
+
+    public function testGreaterThanOrEqual(): void
+    {
+        $i = Integer::from(2);
+        $j = Integer::from(1);
+        $k = Integer::from(2);
+
+        $this->assertTrue($i->greaterThanOrEqual($j));
+        $this->assertTrue($i->greaterThanOrEqual($k));
+    }
+
+    public function testLessThan(): void
+    {
+        $i = Integer::from(2);
+        $j = Integer::from(1);
+        $k = Integer::from(2);
+
+        $this->assertTrue($j->lessThan($i));
+        $this->assertFalse($k->lessThan($i));
+    }
+
+    public function testLessThanOrEqual(): void
+    {
+        $i = Integer::from(2);
+        $j = Integer::from(1);
+        $k = Integer::from(2);
+
+        $this->assertTrue($j->lessThanOrEqual($i));
+        $this->assertTrue($k->lessThanOrEqual($i));
+    }
+
+    public function testMultiply(): void
+    {
+        $i = Integer::from(2);
+        $j = Integer::from(3);
+        $k = Integer::from(6);
+
+        $this->assertTrue($k->equals($i->multiply($j)));
+    }
+
+    public function testDivide(): void
+    {
+        $i = Integer::from(6);
+        $j = Integer::from(3);
+        $k = Integer::from(2);
+
+        $this->assertTrue($k->equals($i->divide($j)));
+    }
+
+    public function testModule(): void
+    {
+        $i = Integer::from(6);
+        $j = Integer::from(3);
+        $k = Integer::from(0);
+
+        $this->assertTrue($k->equals($i->module($j)));
+    }
+
+    public function testToString(): void
+    {
+        $i = Integer::from(1);
+
+        $this->assertSame('1', $i->toString());
+        $this->assertSame('1', (string) $i);
+    }
+}
diff --git a/tests/Model/Integer/PositiveIntegerTest.php b/tests/Model/Integer/PositiveIntegerTest.php
new file mode 100644
index 0000000..36cdd04
--- /dev/null
+++ b/tests/Model/Integer/PositiveIntegerTest.php
@@ -0,0 +1,36 @@
+<?php
+
+declare(strict_types=1);
+
+/*
+ * This file is part of OpenSolid package.
+ *
+ * (c) Yonel Ceruto <open@yceruto.dev>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace OpenSolid\Domain\Tests\Model\Integer;
+
+use OpenSolid\Domain\Error\InvariantViolation;
+use OpenSolid\Domain\Model\Integer\PositiveInteger;
+use PHPUnit\Framework\TestCase;
+
+class PositiveIntegerTest extends TestCase
+{
+    public function testPositiveInteger(): void
+    {
+        $i = PositiveInteger::from(1);
+
+        $this->assertSame(1, $i->value());
+    }
+
+    public function testNegativeInteger(): void
+    {
+        $this->expectException(InvariantViolation::class);
+        $this->expectExceptionMessage('Value must be positive.');
+
+        PositiveInteger::from(-1);
+    }
+}
diff --git a/tests/Model/String/NonEmptyStringTest.php b/tests/Model/String/NonEmptyStringTest.php
new file mode 100644
index 0000000..e980770
--- /dev/null
+++ b/tests/Model/String/NonEmptyStringTest.php
@@ -0,0 +1,42 @@
+<?php
+
+declare(strict_types=1);
+
+/*
+ * This file is part of OpenSolid package.
+ *
+ * (c) Yonel Ceruto <open@yceruto.dev>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace OpenSolid\Domain\Tests\Model\String;
+
+use OpenSolid\Domain\Error\InvariantViolation;
+use OpenSolid\Domain\Model\String\NonEmptyString;
+use PHPUnit\Framework\TestCase;
+
+class NonEmptyStringTest extends TestCase
+{
+    public function testEmptyError(): void
+    {
+        $this->expectException(InvariantViolation::class);
+        $this->expectExceptionMessage('String cannot be empty.');
+
+        NonEmptyString::from('');
+    }
+
+    public function testEmptyWithBlankSpacesError(): void
+    {
+        $this->expectException(InvariantViolation::class);
+        $this->expectExceptionMessage('String cannot be empty.');
+
+        NonEmptyString::from('   ');
+    }
+
+    public function testTrimString(): void
+    {
+        $this->assertSame('A', NonEmptyString::from(' A ')->value());
+    }
+}
diff --git a/tests/Model/String/StrTest.php b/tests/Model/String/StrTest.php
new file mode 100644
index 0000000..d95cc29
--- /dev/null
+++ b/tests/Model/String/StrTest.php
@@ -0,0 +1,89 @@
+<?php
+
+declare(strict_types=1);
+
+/*
+ * This file is part of OpenSolid package.
+ *
+ * (c) Yonel Ceruto <open@yceruto.dev>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace OpenSolid\Domain\Tests\Model\String;
+
+use OpenSolid\Domain\Model\String\Str;
+use PHPUnit\Framework\TestCase;
+
+class StrTest extends TestCase
+{
+    public function testCreate(): void
+    {
+        $s = Str::from('foo');
+
+        $this->assertSame('foo', $s->value());
+    }
+
+    public function testEquals(): void
+    {
+        $s1 = Str::from('foo');
+        $s2 = Str::from('foo');
+        $s3 = Str::from('bar');
+
+        $this->assertTrue($s1->equalsTo($s2));
+        $this->assertFalse($s1->equalsTo($s3));
+    }
+
+    public function testToString(): void
+    {
+        $s = Str::from('foo');
+
+        $this->assertSame('foo', (string) $s);
+        $this->assertSame('foo', $s->toString());
+    }
+
+    public function testLength(): void
+    {
+        $s = Str::from('foo');
+
+        $this->assertSame(3, $s->length());
+    }
+
+    public function testIsEmpty(): void
+    {
+        $s1 = Str::from('foo');
+        $s2 = Str::from('');
+
+        $this->assertFalse($s1->isEmpty());
+        $this->assertTrue($s2->isEmpty());
+    }
+
+    public function testTrim(): void
+    {
+        $s = Str::from(' foo ');
+
+        $this->assertSame('foo', $s->trim()->value());
+    }
+
+    public function testToLower(): void
+    {
+        $s = Str::from('FOO');
+
+        $this->assertSame('foo', $s->lower()->value());
+    }
+
+    public function testToUpper(): void
+    {
+        $s = Str::from('foo');
+
+        $this->assertSame('FOO', $s->upper()->value());
+    }
+
+    public function testToTitle(): void
+    {
+        $s = Str::from('foo bar');
+
+        $this->assertSame('Foo bar', $s->title()->value());
+    }
+}
diff --git a/tests/Model/Uid/UuidV7Base58Test.php b/tests/Model/Uid/UuidV7Base58Test.php
new file mode 100644
index 0000000..be5232f
--- /dev/null
+++ b/tests/Model/Uid/UuidV7Base58Test.php
@@ -0,0 +1,61 @@
+<?php
+
+declare(strict_types=1);
+
+/*
+ * This file is part of OpenSolid package.
+ *
+ * (c) Yonel Ceruto <open@yceruto.dev>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace OpenSolid\Domain\Tests\Model\Uid;
+
+use OpenSolid\Domain\Error\InvariantViolation;
+use OpenSolid\Domain\Model\Uid\UuidV7Base58;
+use PHPUnit\Framework\TestCase;
+
+class UuidV7Base58Test extends TestCase
+{
+    public function testCreate(): void
+    {
+        $id = UuidV7Base58::create();
+
+        $this->assertSame(22, strlen($id->value()));
+    }
+
+    public function testCreateFromString(): void
+    {
+        $id = UuidV7Base58::from('1C4Gx3HwRKMqqm8pYTjiXg');
+
+        $this->assertSame(22, strlen($id->value()));
+        $this->assertSame('1C4Gx3HwRKMqqm8pYTjiXg', $id->value());
+    }
+
+    public function testCreateFromStringInvalid(): void
+    {
+        $this->expectException(InvariantViolation::class);
+        $this->expectExceptionMessage('Invalid UUID: "1C4Gx"');
+
+        UuidV7Base58::from('1C4Gx');
+    }
+
+    public function testEquals(): void
+    {
+        $id1 = UuidV7Base58::from('1C4Gx3HwRKMqqm8pYTjiXg');
+        $id2 = UuidV7Base58::from('1C4Gx3HwRKMqqm8pYTjiXg');
+        $id3 = UuidV7Base58::from('1C4Gx3HwRKMqqm8pYTjiXh');
+
+        $this->assertTrue($id1->equals($id2));
+        $this->assertFalse($id1->equals($id3));
+    }
+
+    public function testToString(): void
+    {
+        $id = UuidV7Base58::from('1C4Gx3HwRKMqqm8pYTjiXg');
+
+        $this->assertSame('1C4Gx3HwRKMqqm8pYTjiXg', (string) $id);
+    }
+}
diff --git a/tests/Model/Uid/UuidV7Rfc4122Test.php b/tests/Model/Uid/UuidV7Rfc4122Test.php
new file mode 100644
index 0000000..520361f
--- /dev/null
+++ b/tests/Model/Uid/UuidV7Rfc4122Test.php
@@ -0,0 +1,61 @@
+<?php
+
+declare(strict_types=1);
+
+/*
+ * This file is part of OpenSolid package.
+ *
+ * (c) Yonel Ceruto <open@yceruto.dev>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace OpenSolid\Domain\Tests\Model\Uid;
+
+use OpenSolid\Domain\Error\InvariantViolation;
+use OpenSolid\Domain\Model\Uid\UuidV7Rfc4122;
+use PHPUnit\Framework\TestCase;
+
+class UuidV7Rfc4122Test extends TestCase
+{
+    public function testCreate(): void
+    {
+        $id = UuidV7Rfc4122::create();
+
+        $this->assertSame(36, strlen($id->value()));
+    }
+
+    public function testCreateFromString(): void
+    {
+        $id = UuidV7Rfc4122::from('f81d4fae-7dec-11d0-a765-00a0c91e6bf6');
+
+        $this->assertSame(36, strlen($id->value()));
+        $this->assertSame('f81d4fae-7dec-11d0-a765-00a0c91e6bf6', $id->value());
+    }
+
+    public function testCreateFromStringInvalid(): void
+    {
+        $this->expectException(InvariantViolation::class);
+        $this->expectExceptionMessage('Invalid UUID: "f81d4fae".');
+
+        UuidV7Rfc4122::from('f81d4fae');
+    }
+
+    public function testEquals(): void
+    {
+        $id1 = UuidV7Rfc4122::from('f81d4fae-7dec-11d0-a765-00a0c91e6bf6');
+        $id2 = UuidV7Rfc4122::from('f81d4fae-7dec-11d0-a765-00a0c91e6bf6');
+        $id3 = UuidV7Rfc4122::from('f81d4fae-7dec-11d0-a765-00a0c91e6bf7');
+
+        $this->assertTrue($id1->equals($id2));
+        $this->assertFalse($id1->equals($id3));
+    }
+
+    public function testToString(): void
+    {
+        $id = UuidV7Rfc4122::from('f81d4fae-7dec-11d0-a765-00a0c91e6bf6');
+
+        $this->assertSame('f81d4fae-7dec-11d0-a765-00a0c91e6bf6', (string) $id);
+    }
+}