Four-wave mixing (FWM) is one of the best known phenomena in semiconductor optics. Recent experimental results of FWM instabilities and optical switching in atomic systems have renewed the interest in FWM and possible related instabilities in semiconductors. We have recently performed theoretical investigations of FWM instabilities in a variety of semiconductor quantum well systems (single quantum wells, Bragg-spaced multiple quantum wells, and planar semiconductor micocavities) and shown that different systems require different physical processes that potentially can give rise to FWM instabilities. In this contribution, we concentrate on the simple (and largely academic) finding that phase-space filling together with spatial exciton dispersion can lead to FWM instabilities in single quantum wells. © 2009 WILEY-VCH Verlag GmbH & Co. KGaA.