The presence or absence of turbulence in near-bed flows is known to affect patterns of sediment transport. Therefore the accurate prediction of turbulence is important when considering the initiation of motion of medium to coarse sands as such grains may be eroded by conditions similar to those causing turbulence. An experimental study is described to investigate boundary layer stability under nonlinear (asymmetric) oscillatory flows. In addition, the results are combined with data collected elsewhere to better explain the dependence of initial transition to turbulence on bed roughness and oscillatory period. An oscillating trolley system simulating such oscillatory flows at prototype scale is described. The transition to turbulence under asymmetric (nonlinear) oscillatory flow is investigated utilizing visual techniques and the high-resolution measurement of near-bed velocity. These data are used in conjunction with the extensive data sets of Li <1954> and Manohar <1955> for linear oscillatory flows, collected using similar equipment and methods. Relationships are presented to describe the transition over smooth and fixed (granular) roughness beds under linear and nonlinear flows. The transition to turbulence is shown to have a strong positive linear correlation with both oscillatory period and grain roughness up to a maximum Reynolds number for linear flows. An additional positive nonlinear relationship with asymmetry is observed; this was proportionately greater with increasing bed roughness (from smooth beds to a uniform bed roughness of 550 ¿m). These relationships suggest that transition is regulated by a balance between stabilizing and destabilizing forces or conditions, namely, rates of fluid acceleration, the time frame for development of turbulence, and sources of the initial flow perturbation. It is argued that (wave) period dependence observed in threshold of motion data may be explained by a combination of (a) period dependency in the transition to turbulence or of (b) translation of the proposed regulatory mechanisms for the transition to turbulence.