A biosensor is a device which converts biological activity into a quantifiable signal. The basic principles, operation and applications of some typical electrochemical, piezo-electric and optical devices are reviewed. These sensors promise to provide an analytically powerful and inexpensive alternative to conventional technologies by enabling the identification of target substances in the presence of a number of interfering species. In the extreme cases where this is not possible, an integrated micro-separation and detection system on a silicon wafer is proposed. Finally, a summary of future markets and prospects for the developing technology of biosensing is given, with particular reference to growth opportunities in the healthcare sector.