The fact that histones are modified by acetylation has been known for almost 30 years. The recent identification of enzymes that regulate histone acetylation has revealed a broader use of this modification than was suspected previously. Acetylases are now known to modify a variety of proteins, including transcription factors, nuclear import factors and alpha-tubulin. Acetylation regulates many diverse functions, including DNA recognition, protein-protein interaction and protein stability. There is even a conserved structure, the bromodomain, that recognizes acetylated residues and may serve as a signalling domain. If you think all this sounds familiar, it should be. These are features characteristic of kinases. So, is acetylation a modification analogous to phosphorylation? This review sets out what we know about the broader substrate specificity and regulation of acetyl- ases and goes on to compare acetylation with the process of phosphorylation.