Michael Gaster’s work focuses on the mechanism of transition in a boundary layer. He elucidated the relationship between temporal and spatial instabilities by means of a group velocity and made a quantitative prediction of the development of waves in growing boundary layers. He has measured the evolution of localised wave packets and demonstrated a substantial measure of agreement with his theory. He has shed valuable light on aspects of leading edge separation on thin wings, especially on the question of transition to turbulence and reattachment. He has also made important contributions to problems of vortex shedding, turbulent wedges and the spectral analysis of random signals. His work is marked by a unification of theoretical ideas with experiment, partly brought about by ingenious use of computational methods.