A two–dimensional separation bubble on a flat plate is studied experimentally by means of hot–wire anemometry and flow visualization. Separation of the laminar boundary layer on the plate is caused by an adverse pressure gradient imposed by a curved wall opposite to the plate. The instability of, and transition process in, the separation bubble are focused on. The bubble is found to be highly susceptible to high–frequency two–dimensional instability waves, which are studied under both natural and forced conditions. A similar development of these instability waves in the separation bubble is found in both cases. The exponential growth of the two–dimensional disturbances dominates the flow except for in the reattachment region, where large–scale three–dimensional structures appear. Some difficulties associated with experimental investigations of boundary–layer separation–bubble flows are discussed.