A biosensor can be defined as, a compact analytical device incorporating a biological or biologically derived sensing element (such as an enzyme ,antibody, microbe or DNA) either integrated within or intimately associated with a physicochemical transducers. Upon interaction with a chemical species, the physicochemical properties of the sensing layer (mass, optical properties & resistance) change & this is detected by the transducer. The changes are then converted into an electrical signal which is then processed. The transducer may be optical (optical fibre), electrochemical (ion- selective electrodes) or heat-sensitive. Biosensors are making a great impact on the development of rapid, sensitive assays for the detection of microorganisms. Although much success has been achieved in terms of research, commercial development has been slow. Kits are now available for several organisms such as E.coli O157:H7 & Salmonella typhimurium and it is hoped that more will become available shortly. New developments include integrated systems and the use of molecular beacons & nanosensor production. These should ensure even more rapid and specific detection.