The life span of stressed components operating at high temperature is limited by such factors as excessive deformation, rupture, fatigue and brittle fracture, but in this report only the role of deformation is discussed.In components which creep, the deformation is normally limited so that the average creep strain allowed is 1%, or about ten times the maximum elastic strain, while the maximum allowable strain is limited to 5%.Under constant loading, the determination of the creep strains is a fairly straightforward procedure, although for complex geometries the computing power required can be considerable.It is under conditions of cyclic loading, however, that the determination of the creep strains becomes more difficult, and indeed it is because of the cyclic loading that deformation rates can sometimes accelerate very dramatically.