The thermal characteristics of power transistors and their measurement are discussed. The devices discussed include bipolar transistors and metal-oxide-semiconductor field-effect transistors (MOSFETs). Measurement problems common to these devices are addressed, such as general methods for measuring device temperature, control of thermal environment, selection of a temperature-sensitive electrical parameter, measurement of temperature-sensitive electrical parameters, reasons for measuring temperature, and temperature measurement of integrated power devices. Procedures for detecting nonthermal switching transients, extrapolation of the measured temperature to the instant of switching, and for measuring the temperature of Darlington transistors are included. The needs for thermal characterization of evolving devices such as high voltage and power integrated circuits and merged bipolar/MOSFET devices are mentioned.