The pronator quadratus muscle has been neglected to a great extent in the anatomical and functional literature. This study confirms the consistent presence of two distinct heads to the muscle, and defines the gross and microscopic anatomy. The functional significance of these heads has been studied using in vitro and in vivo techniques to demonstrate that the superficial head is the prime mover in forearm pronation, and that the deep head is a dynamic stabilizer of the distal radioulnar joint.