The noise performance of transistor mixers at high frequencies is investigated by first considering the signal processes in the mixer. A knowledge of the mixer signal performance, and consideration of the various noise sources in the transistor, allows calculation of expressions for the mixer noise figure and optimum source resistance. The effects of variation in operating conditions are discussed, and, in particular, it is shown that, at very high frequencies, the mixer noise figure increases rapidly with collector current. Expressions for mixer and amplifier noise figure are compared and the basic differences are discussed.