The development of biochemical detection systems that use antibodies as the recognition element, also known as immunosensors, has generated considerable interest from science and industry alike. This is due to the practically inexhaustible number of applications for which such a technology could conceivable lend itself, such as medicine, process control, and environmental monitoring. The potentially high sensitivity of such systems makes them ideal for situations where the utmost speed and accuracy are demanded. Moreover, there is theoretically no limit to the chemical and biological moieties for which an immunosensor can be manufactured. This critical review emphasizes both the biological and transduction aspects of optical immunosensors. The basic concepts of antibody-antigen interactions in solution are reviewed, and factors affecting the kinetics at the solid-liquid interface are characterized. In addition, the aspects of reagent immobilization, inherent to immunosensor design, are discussed. Finally, a comprehensive overview describing the mechanisms, advantages, and shortcomings of several immunosensing formats such as fiber-optic, planar waveguide, surface plasmon resonance, and continuous-flow immunosensor is presented. The future of these sensors, we well as the integration of fluorescence lifetime sensing and fluorescent polarization technologies with immunosensing technologies, is contemplated, posing the question: What place will immunosensors secure in tomorrow's market?