Transcription repression mediated through histone deacetylase (HDAC) complexes is widespread, and mechanisms by which HDAC complexes act have been revealed by extensive studies in vitro and in cell culture. However, until recently, little has been known about the developmental roles of histone deacetylation. Mutants now exist for a number of members of the two major HDAC complexes (NuRD and SIN3) and some associated proteins. The emerging picture is that these complexes have specific functions in development, rather than being required for most cellular processes.