Experiments on turbulent boundary were layers subjected to controlled unsteadiness were performed in a special water channel. The flow is steady in the development section upstream of the unsteady test section where the boundary layer is subjected to an oscillating adverse free-stream velocity gradient sufficient to induce flow reversal near the wall. Measurements of the mean, oscillatory, and turbulence components of the streamwise velocity in the boundary layer indicate that the mean velocity and mean turbulent stresses are unaffected by the oscillations, whereas the periodic components of these quantities are strongly dependent upon frequency. At low frequencies the boundary layer behaves quasistaticly at intermediate frequencies the boundary layer behavior correlates with the Strouhal number based on the length of the unsteady region and at high frequencies the outer region of the boundary layer moves as a slug while the sublayer behaves as a Stokes layer described by laminar equations. Reverse flows occur in this Stokes layer, but the boundary layer remains thin and hence attached to the surface.