A helmet, particularly an aircrew helmet, has a rear part (10) and a front part (12). The rear part (10) includes a shell shaped to extend partially over the top of the wearer's head and to each side of the head. The front part (12) is shaped to fit against the rear part (10) to complete the shell of the helmet, and is detachably connected to the rear part. The front part (12) is shaped to accommodate equipment for optical protection or enhancement, such as a visor or night vision goggles. A single helmet may have two or more interchangeable front parts with different optical equipment. The front part (12) may be connected to the rear part (10) by releasable catches (50, 54) at the top and sides of the helmet. The catch (50) at the top may allow the front part (12) to pivot upwards to enable the helmet to be donned and doffed without detaching the front part.