Electrolyte-gated organic field-effect transistors have emerged in the field of biosensors over the last five years, due to their attractive simplicity and high sensitivity to interfacial changes, both on the gate/electrolyte and semiconductor/electrolyte interfaces, where a target-specific bioreceptor can be immobilized. This article reviews the recent literature concerning biosensing with such transistors, gives clues to understanding the basic principles under which electrolyte-gated organic field-effect transistors work, and details the transduction mechanisms that were investigated to convert a receptor/target association into a change in drain current.