In recent years calcium has become recognized as an important 'second messenger', in that an increase in free intracellular calcium ion concentration is involved in many aspects of cellular activation. In excitable cells such as smooth muscle or cardiac tissue an influx of extracellular calcium ions through voltage sensitive calcium channels plays a major role in increasing the cytoplasmic free calcium concentration. It is now known that the activity of these calcium channels may be inhibited or stimulated by a range of ions, toxins and drugs. This article outlines the biochemistry and pharmacology of these 'calcium modulators', and discusses their present and future role as both biological tools, and as potent and selective drugs.