SYNOPSIS. The locomotion and orientation of free-swimming microorganisms have been widely studied for more than a century. With few exceptions, only the two-dimensional translational velocity of the organism in question is ever reported, yet a complete description of motion requires a three-dimensional description of both the translational and rotational velocities. Recent theoretical work, considering both the translational and rotational velocities, has demonstrated that a previously unrecognized orientation mechanism exists—helical klinotaxis. Efforts to test the predictions of this theory are described, concluding that helical klinotaxis is probably utilized by a diverse assemblage of free-swimming microorganisms.