The document summarizes ion sensitive field effect transistors (ISFETs) and their potential applications as biosensors. It discusses the basic principles of ISFET operation, which involves a chemically sensitive insulator layer exposed to an electrolyte solution. Changes in ion concentration in the solution, such as hydrogen ions (pH), alter the surface charge of the insulator layer and thus the threshold voltage of the ISFET. The document reviews the modeling of ISFETs based on metal oxide semiconductor field effect transistor (MOSFET) theory. It also describes site-binding and electrical double layer theories that explain the mechanism by which the insulator surface charge responds to ion concentration. Finally, the document notes the commercialization of some ISFET-