A series of direct numerical simulations (DNS) of the flow past a zero pressure gradient flat plate with rows of dimples is carried out. The Reynolds number based on the boundary layer thickness is 1000 and the dimples have a circular profile with a depth to diameter ratio of 0.1. The incoming flow is laminar and the ratio of the incoming boundary layer thickness to the dimple depth determines the critical Reynolds number, where transition to turbulence occurs downstream. This happens as the shear layer that forms at the dimple edge separates over the first row of dimples and becomes unstable creating coherent vortex sheet. The vortex sheet undergoes a complex spanwise instability transforming themselves into a packet of horseshoe vortices. As a result the boundary layer downstream of the dimples has the same qualitative features encountered in wall bounded turbulent boundary layers.