The design of most aerodynamic surfaces, as for example the helicopter rotor, is based essentially on quasi-steady theories. However the dynamics of a rotating blade introduce unexpected fluctuations and overshoots of properties like lift, drag, etc. The phenomenon of unsteady stall is intimately connected with the development of an oscillating boundary layer and separation. Experimental investigation of such flows was undertaken by a method of visualization developed especially for the study of laminar or turbulent boundary layers and separation. The method captures the instantaneous 2-D flow field, including regions of separated flow and provides accurate quantitative information. Laser doppler anemometer measurements complement the optically received data. Results reveal that separation responds with time-lag to external disturbances, in agreement with unsteady stall data. Oscillating outer flows result in displacement of the point of separation and under certain conditions, the Despard and Miller criterion was found to hold. Earlier theoretical models of separation are confirmed qualitatively and for the early stages of the transient phenomena. The findings provide physical insight and quantitative data that may help understand the phenomenon of unsteady stall and unsteady separation. (Author).