Abstract As an introduction to this book, a variety of semiconductor gas sensors is presented. They are classified into five types, according to the transducers used: resistor, diode, MIS (metal–insulator–semiconductor) capacitor, MIS FET (metal–insulator–semiconductor field-effect transistor), and oxygen concentration cell. The structure, working principle, and sensing mechanism are described for each type. As the only type successfully existing on the market at present, the resistor is allotted considerably more space than the alternative semiconductor gas sensors, with experimental knowledge and the theory of gas response described in detail. The receptor function of metal oxides recently found in resistors is also of critical importance in other types incorporating the same oxides; the contact potential generated between oxide and metal is likely a main origin of the gas response of those devices. Finally, observations on the prospects for and problems with semiconductor gas sensors are made.