diff --git a/QueryGenerator.php b/QueryGenerator.php
index 220c43b..9504a64 100644
--- a/QueryGenerator.php
+++ b/QueryGenerator.php
@@ -11,7 +11,7 @@
  *
  * Available clauses / methods are:
  *  select, insert, replace, update, delete, from, join, set, columns, values,
- *  where, group, having, order, limit, offset, duplicate, modify
+ *  where, group, having, union, order, limit, offset, duplicate, modify
  *
  * After all clauses have been built, call the 'build' member function to
  * compose the entire query. This returns an array containing the query and
@@ -136,6 +136,12 @@ class QueryGenerator {
          'suffix' => ')',
          'requiresArgument' => true,
       ],
+      'union' => [
+         'clause' => '',
+         'prefix' => '',
+         'glue' => "\n",
+         'suffix' => '',
+      ],
       'order' => [
          'clause' => 'ORDER BY ',
          'prefix' => '',
@@ -195,7 +201,7 @@ class QueryGenerator {
     * @var non-empty-array<string, list<string>>
     */
    private static array $possibleClauses = [
-      'select' => ['from', 'join', 'where', 'group', 'having', 'order', 'limit', 'offset', 'forupdate'],
+      'select' => ['from', 'join', 'where', 'group', 'having', 'union', 'order', 'limit', 'offset', 'forupdate'],
       'insert' => ['set', 'columns', 'values', 'duplicate', 'as'],
       'replace' => ['set', 'columns', 'values'],
       'update' => ['set', 'where', 'order', 'limit'],
@@ -299,6 +305,39 @@ public function __call(string $method, array $args) {
       return $this;
    }
 
+   /**
+    * Append a UNION operand rendered as a complete SELECT statement.
+    * QueryGenerator operands are built (and validated, unless the operand
+    * has called skipValidation) immediately.
+    */
+   public function union(self|string $query, mixed $params = []): self {
+      return $this->addUnion('UNION', $query, $params);
+   }
+
+   public function unionAll(self|string $query, mixed $params = []): self {
+      return $this->addUnion('UNION ALL', $query, $params);
+   }
+
+   private function addUnion(string $keyword, self|string $query, mixed $params): self {
+      if ($query instanceof self) {
+         if ($params !== []) {
+            throw new Exception(
+               "Params can't be passed alongside a QueryGenerator operand; " .
+               "add them to the operand instead."
+            );
+         }
+         [$query, $params] = $query->build();
+      }
+
+      if (!is_array($params)) {
+         $params = [$params];
+      }
+
+      $this->clauses['union'][] = "$keyword\n$query";
+      $this->params['union'] = array_merge($this->params['union'], $params);
+      return $this;
+   }
+
    /**
     * Combine the clauses and parameters in this QueryGenerator to compose a
     * complete query and paramter list.
@@ -313,7 +352,7 @@ public function __call(string $method, array $args) {
     *
     * Returns an array containing the query and paramter list, respectively.
     *
-    * @return array{0: string, 1: array<string, mixed>}
+    * @return array{0: string, 1: list<mixed>}
     */
    public function build(bool $skipClauses = false): array {
       if ($this->validateQuery) {
@@ -338,7 +377,7 @@ public function build(bool $skipClauses = false): array {
          $clauses[] = $this->constructClause($method, $skipClauses);
          $params = array_merge($params, $this->params[$method]);
       }
-      return [implode("\n", $clauses), $params];
+      return [implode("\n", $clauses), array_values($params)];
    }
 
    /**
diff --git a/QueryGeneratorTest.php b/QueryGeneratorTest.php
index b19850c..813a04b 100644
--- a/QueryGeneratorTest.php
+++ b/QueryGeneratorTest.php
@@ -241,6 +241,144 @@ public function testOnDuplicate(): void {
       $this->assertQuery($qGen, $expectedQuery, []);
    }
 
+   public function testUnion(): void {
+      $other = new QueryGenerator();
+      $other->select('field');
+      $other->from('table2');
+
+      $qGen = new QueryGenerator();
+      $qGen->select('field');
+      $qGen->from('table');
+      $qGen->union($other);
+
+      $expectedQuery = <<<EOT
+SELECT field
+FROM table
+UNION
+SELECT field
+FROM table2
+EOT;
+
+      $this->assertQuery($qGen, $expectedQuery, []);
+   }
+
+   public function testUnionAll(): void {
+      $other = new QueryGenerator();
+      $other->select('field');
+      $other->from('table2');
+
+      $qGen = new QueryGenerator();
+      $qGen->select('field');
+      $qGen->from('table');
+      $qGen = $qGen->unionAll($other);
+
+      $expectedQuery = <<<EOT
+SELECT field
+FROM table
+UNION ALL
+SELECT field
+FROM table2
+EOT;
+
+      $this->assertQuery($qGen, $expectedQuery, []);
+   }
+
+   public function testMixedUnions(): void {
+      $qGen = new QueryGenerator();
+      $qGen->select('a');
+      $qGen->from('b');
+      $qGen->union('SELECT a FROM c');
+      $qGen->unionAll('SELECT a FROM d');
+
+      $expectedQuery = <<<EOT
+SELECT a
+FROM b
+UNION
+SELECT a FROM c
+UNION ALL
+SELECT a FROM d
+EOT;
+
+      $this->assertQuery($qGen, $expectedQuery, []);
+   }
+
+   public function testUnionStringOperand(): void {
+      $qGen = new QueryGenerator();
+      $qGen->select('id');
+      $qGen->from('live');
+      $qGen->union('SELECT id FROM archive WHERE x = ?', 5);
+
+      $expectedQuery = <<<EOT
+SELECT id
+FROM live
+UNION
+SELECT id FROM archive WHERE x = ?
+EOT;
+
+      $this->assertQuery($qGen, $expectedQuery, [5]);
+   }
+
+   public function testUnionParamOrdering(): void {
+      $other = new QueryGenerator();
+      $other->select('a');
+      $other->from('c');
+      $other->where('y = ?', 2);
+
+      $qGen = new QueryGenerator();
+      $qGen->select('a');
+      $qGen->from('b');
+      $qGen->where('x = ?', 1);
+      $qGen->union($other);
+      $qGen->order('a');
+      $qGen->limit(10);
+      $qGen->offset('?', 5);
+
+      $expectedQuery = <<<EOT
+SELECT a
+FROM b
+WHERE (x = ?)
+UNION
+SELECT a
+FROM c
+WHERE (y = ?)
+ORDER BY a
+LIMIT 10
+OFFSET ?
+EOT;
+
+      $this->assertQuery($qGen, $expectedQuery, [1, 2, 5]);
+   }
+
+   public function testUnionGeneratorOperandWithParamsThrows(): void {
+      $other = new QueryGenerator();
+      $other->select('a');
+      $other->from('c');
+
+      $qGen = new QueryGenerator();
+      $qGen->select('a');
+      $qGen->from('b');
+
+      $this->expectException(Exception::class);
+      $qGen->union($other, [1]);
+   }
+
+   public function testUnionOperandValidation(): void {
+      $incomplete = new QueryGenerator();
+      $incomplete->select('a');
+
+      $qGen = new QueryGenerator();
+      $qGen->select('a');
+      $qGen->from('b');
+
+      $union = function() use ($qGen, $incomplete): QueryGenerator {
+         return $qGen->union($incomplete);
+      };
+
+      $this->assertTrue((bool)$this->didThrowException($union));
+      $incomplete->skipValidation();
+      $this->assertFalse($this->didThrowException($union));
+   }
+
    public function assertQuery(QueryGenerator $qGen, string $expectedQuery, array $expectedParams): void {
       [$actualQuery, $actualParams] = $qGen->build();
       $this->assertEquals($expectedQuery, $actualQuery);
diff --git a/README.md b/README.md
index 15569e9..024c44e 100644
--- a/README.md
+++ b/README.md
@@ -84,7 +84,7 @@ WHERE field1 > ? AND field3 < ?
 
 QueryGenerator has support for SELECT, INSERT, REPLACE, UPDATE, and DELETE
 queries. Each of those query types supports a selection of different clauses:
- * select: from, join, where, group, having, order, limit, offset, forupdate
+ * select: from, join, where, group, having, union, order, limit, offset, forupdate
  * insert: set, columns, values, duplicate
  * replace: set, columns, values
  * update: set, where, order, limit
@@ -92,3 +92,24 @@ queries. Each of those query types supports a selection of different clauses:
 
 Simply call the member function of the clause you want to add to, passing
 strings of SQL and (optionally) parameters for use in prepared statements.
+
+SELECT queries can be combined with `union()` and `unionAll()`. Operands
+must be complete SELECT statements, either another QueryGenerator or a raw
+SQL string:
+
+```php
+$archived = new QueryGenerator();
+$archived->select('id')->from('archived_orders')->where('total > ?', 100);
+
+$qGen = new QueryGenerator();
+$qGen->select('id')->from('orders')->where('total > ?', 100);
+$qGen->union($archived);
+$qGen->unionAll('SELECT id FROM pending_orders');
+$qGen->order('id');
+```
+
+A trailing `order()` or `limit()` applies to the whole union result. To
+order or limit an individual operand, pass it as a parenthesized string.
+
+QueryGenerator operands are rendered when `union()` or `unionAll()` is
+called; mutating the operand afterward has no effect on the outer query.