Calcium is thought to be involved in regulating mitotic transitions. The basis for this view is set out. Recent data from experiments on sea urchin embryos is discussed. The relative simplicity of the embryonic cell cycle and the relative ease with which cell physiology can be done in sea urchin embryos has allowed the clear demonstration that the phosphoinositide-calcium-calmodulin signalling pathway is required for and regulates mitosis entry and anaphase onset. The relevance of the sea urchin work to mitosis in other cell types is briefly discussed.