High temperature exhaust gas sensors have been used since 1975 to monitor catalyst overtemperature in the Japan auto market. Heated catalyst systems introduced to meet LEV and ULEV will make use of high temperature sensors for control and diagnosis. This paper describes the history and the evolution of these sensors. The three leading sensor technologies consisting of thermistors, thermocouples, and resistive temperature detectors (RTD) are described. Interface and system requirements for each of the sensor technologies are summarized. Sensor response time characteristics interaction with system design are covered.