The history of the problem of temper brittleness is surveyed; the reviews of Hollomon, Woodfine, and Low are reexamined in the light of recent developments, and the work carried out since 1959 is reviewed. Included in the latter is a model which has been proposed to explain the phenomenon. The problem of “500 F embrittlement” and its connection with temper brittleness are discussed, and a rudimentary model is proposed. Suggested directions for future research are indicated.