Over the past few years there has been a resurgance of interest in the cell cycle. The excitement has mainly been due to the fact that researchers all over the world who had been working on seemingly different processes in yeast, fruit flies, frogs and man have found that many of the processes and individual proteins have been highly conserved. In essence, what regulates the cell cycle in yeast also works in man. This review is biased towards cell cycle regulation in mammalian cells but we have also covered what is known about similar mechanisms in other organisms to provide a broader perspective to the field. We outline what is currently known about the cel cycle and the key points at which cell prolifertion is controlled. We summarise recent work on cell cycle control genes and antioncogenes and the post-transcriptional regulation of the proteins for which they code. Finally we cover the relationship between the cell cycle and differentiation.