Several examples of three-dimensi onal boundary layers on helicopter rotors in forward flight are analyzed within the framework of small-crossflow and quasi-steady approximations; these two assumptions are shown to be mutually consistent and to have a wide range of applicability. In laminar flow, the crossflow can be separated into components due to rotation and due to the instantaneous yawing of the rotor blade relative to the direction of flight. Third-order perturbation expansions for a flat-plate blade show the relative importance of the rotational and translational motion, and analysis of the flow over several blade sections with thickness demonstrates the role of chord wise pressure gradients. The results indicate that the crossflow generally tends to delay separation on the retreating side of the rotor disk. The effects of rotation can be large in regions of incipient separation, but elsewhere the boundary layer generally resembles the viscous flow over a swept wing. Also, the detailed structure of the crossflow depends upon whether the primary flow is accelerating or decelerating.