Optical sensors are prepared by attaching an indicating layer to the distal tip of an optical fiber. Light is sent down the optical fiber and is modulated by the indicating layer. The degree of modulation of the returned light is a direct measure of the amount of a particular chemical species in contact with the fiber tip. A variety of sensor designs can be employed along with a diverse array of supporting instrumentation. An overview of the various optical transduction mechanisms, instrumentation, and problems associated with optical sensors for continuous clinical monitoring is presented.>