Biosensors are attracting the attention of manyinvestigatorsin the field of biotechnology related research. A biosensoris a device that combinesthe specificity of a biomaterial (enzymes, antibodies, receptors or even whole cells) with sensitive, well established bioassays. Biosensors are of potential use in manyfields includingclinical and biotechnology monitoring. An importantclass of enzymes, effecting H,O, generation or O7 depletion, can be monitored by following the enzymatic reaction, and thus determining the substrate level. Other enzyme systems, like urease andpenicillinase,result in a pH change which can be followed using a pH sensitive device. For the many advantagesoffiber optic sensing, extensive research efforts have been devoted to the productionofreliable,selfcontained andsensitive fiber optic biosensors which are capable of in situ monitoring. An overview ofthe possibilities and challenges associated with the construction of such sensors are discussed.