The coercivity mechanism of precipitation hardened SmCo permanent magnets is dominated by the pinning of magnetic domain walls on the precipitation structure. In this chapter the microstructure of these magnets is determined using transmission electron micrographs and simplified models are developed. The basics of the pinning mechanism on planar interfaces, intercellular phases and a simple cell structure are investigated using micromagnetic simulations and compared with an analytical model. Then, a model of the rhomboidal cell structure is used to study attractive and repulsive pinning for different material parameters and material compositions.