Movement is a basic manifestation of animal life. Yet its expressions and forms are extremely variable in the animal kingdom. It is the diversity of muscles that makes possible such a broad spectrum of movements, fast and slow, covering a more than a million fold speed range. Think, for instance, of the racing of a horse, the buzzing of a bee, or the steady beating of your own heart; and what a contrast between the leap of a gazelle and the sluggish movements of a snail. But there is unity in this diversity. In all muscle studied so far, contraction appears to be basically an interaction of two proteins, actin and myosin, which is more or less precisely controlled by the intracellular calcium ion concentration. This principle of calcium control is so essential for muscle contraction and cell motility that it has been conserved during evolution for more than a billion years. How diverse, on the other hand, are the mechanisms controlling the intracellular calcium ion concentration, the intracellular calcium target proteins and the way in which calcium ions activate the contractile system! For instance, in fast skeletal muscle calcium is handled entirely within the cell by the sarcoplasmic reticulum, whereas in smooth and cardiac muscle the extracellular calcium supply seems to be quite important, In vertebrate skeletal and heart muscle calcium regulates contraction by operating the protein switch troponin, whereas in the much slower smooth muscles calcium influences the myosin molecule either directly (molluscan smooth muscle) or indirectly through an enzymic cascade involving calmodulin and myosin phosphorylation (vertebrate smooth muscle).