This paper deals with the application of simulated annealing to rock magnetism. The implementation of the algorithm is discussed in detail. It is shown that the technique can be used to determine stable micromagnetic structures by finding very low energy states which are not easily found using other optimization techniques. A modified simulated annealing algorithm is used to incorporate thermal fluctuations into a micromagnetic model in order to simulate the acquisition of a thermoremanent magnetization (TRM). The results obtained agree with N¿el's single domain (SD) TRM theory when the magnetization of the model is constrained to rotate coherently. When this constraint is removed, a decrease in the blocking temperature of the model is observed above ≈0.07 μm, showing N¿el's SD TRM theory to be inaccurate for grains above this size. ¿ American Geophysical Union 1994