The creep rate for steels at constant stress and elevated temperature is composed of (1) the strain hardening with a rate which varies inversely with time, and (2) an asymptotic constant creep rate. The sum gives the creep rate at any time, and the formula for the total creep-extension curve. For steels with a physical or structural change during creep, a third quantity must be added to the creep-extension formula, the test value of the quantity indicating the degree of instability of the test metal. To illustrate four of the principal types of instability, the author presents four sets of long-time creep tests extending from 5500 hr to 5 yr. They represent changes due to (1) carbide spheroidization, (2) ferritic banding, (3) dendrites, and (4) alloy segregations.