Calorimetric sensing or thermometric sensing involving immobilized biocatalysts has diversified into several areas of apphcatlon since its introduction in the early 1970s. In principle, a chemical or biological process is monitored and quantified by the changes in the thermal signatures of the reacting species. From the fundamental laws in nature governing molecular reactions, virtually all reactions are associated by the absorption or evolution of heat. It was recognized quite early that most enzymatic reactions are associated with the liberation of heat. This led to the development of several generations of calorimeters that monitored biological reactions and later were successfully applied to the studies on immobilized enzymes. The different generations of calorimeters also used different approaches for heat measurement, such as isothermal, heat-conduction, and isoperibol calorimeters (see Chapter 13, Methods in Biotechnology, vol. 6).