Mammalian cells in vivo frequently express primary cilia. Although some fully differentiated cell types rarely, if ever, express them, most do, indicating that they are regular cell organelles. Their expression can also be explored in vitro, where conditions--physical and chemical, intrinsic and extrinsic--permit experimental approaches which give far greater control than in vivo. This 'state of the art' paper covers briefly the general biology of primary cilia, highlights the current situation with regard to our understanding of their relevance and importance in cell biology from various facets of our recent research, much of it in collaboration with other laboratories world-wide, and outline future work aimed at answering some basic and applied questions about them, within a context of an increasing awareness that signalling between cells is of the utmost importance in understanding proliferation control and its value in cancer research, the major remit of this unit.