When a material like steel is plastically deformed at ambient temperatures its strength is increased due to work hardening. This work hardening effectively prevents any further deformation from taking place if the stress remains approximately constant. Annealing the deformed steel at an elevated temperature removes the work hardening and restores the steel to its original condition. However, if the steel is plastically deformed at an elevated temperature, then both work hardening and annealing take place simultaneously. A consequence of this is that steel under a constant stress at an elevated temperature will continuously deform with time, that is, it is said to "creep".

Creep in steel is important only at elevated temperatures. In general, creep becomes significant at temperatures above about 0.4Tm where Tm is the absolute melting temperature. However, materials having low melting temperatures will exhibit creep at ambient temperatures. Good examples are lead and various types of plastic. For example, lead has a melting temperature of 326^o C (599K) and at 20^o C (293K, or about 0.5Tm) it exhibits similar creep characteristics to those of iron at 650^o C.