An experimental investigation was undertaken to reveal the characteristic flow structure in the regime of nonlinear boundary layer instability and onset of turbulence. A controlled Tollmien-Schlichting wave was introduced into a two-dimensional boundary layer over a flat plate to study the growth and evolution of the controlled disturbances using both hot film measurements and an improved hydrogen bubble visualization technique. The results demonstrate that the actual breakdown of the laminar boundary layer and the breakdown of the L vortices do not imply the immediate onset of turbulence. Rather, the onset occurs later with the breakdown of the long streaks. Therefore, an alternative transitional scenario was developed.