For many materials, primary creep can be described with the phi (ϕ) model and tertiary creep with the Omega (Ω) model (discussed in Chap. 12 ). According to the phi model, the creep rate is linear in strain and time in a double logarithmic diagram. When using empirical descriptions of the creep curves, these models are recommended. Several basic models for primary creep are derived. They are based on the creep rate in the secondary stage. This means that primary creep can be derived without any new data. The primary creep models are in agreement with the phi model and can describe experimental data. For the martensitic 9–12% Cr steels at least two dislocation densities are needed to represent primary creep because the initial dislocation density is high contrary to the situation for annealed fcc materials.