The creep-fatigue properties of steam turbine materials such as the 1%CrMoV steel traditionally adopted for steam inlet temperatures up to ~565°C, the newer advanced 9-11%Cr steels for applications up to ~600°C, and the nickel based Alloy 617 for potential use to >700°C are reviewed, in particular with reference to their cyclic/hold test crack initiation endurances. The results of cyclic/hold creep-fatigue tests are commonly employed to establish the damage summation diagrams used to form the basis of a number of creep-fatigue assessment procedures, and it is demonstrated that care should be exercised in the way such diagrams are interpreted to compare the creep-fatigue resistances of different alloy types. The form of such damage diagrams is dependent, not only on the analytical procedures used to define the respective fatigue and creep damage fractions, but also on both the deformation and damage interaction mechanisms displayed by the material.