Unsteady boundary layers are investigated very actively today as a part of the effort to fully understand and calculate unsteady flows and their applications. As a result a number of review articles have appeared recently. The present review is a supplement to earlier contributions. Emphasis is given here to areas in which considerable progress has been achieved recently. However, in most of the interesting areas today, as for example in experimental investigation of unsteady turbulent boundary layers, modeling of unsteady separation and separated flows, no particular program has been completed. This review article, therefore, is a kind of a unified progress report which will hopefully capture the spirit of the work in progress in different countries of the western world and will identify potential difficulties and future needs.