Many reactions of transition metal compounds involve a change in spin. These reactions may proceed faster, slower—or at the same rate as—otherwise equivalent processes in which spin is conserved. For example, ligand substitution in [CpMo(Cl)₂(PR₃)₂] is faster than expected, whereas addition of dinitrogen to [Cp*Mo(Cl)(PMe₃)₂] is slow. Spin-forbidden oxidative addition of ethylene to [Cp*Ir(PMe₃)] occurs competitively with ligand association. To explain these observations, we discuss the shape of the different potential energy surfaces (PESs) involved, and the energy of the minimum energy crossing points (MECPs) between them. This computational approach is of great help in understanding the mechanisms of spin-forbidden reactions, provided that accurate calculations can be used to predict the relevant PESs. Density functional theory, especially using gradient-corrected and hybrid functionals, performs reasonably well for the difficult problem of predicting the energy splitting between different spin states of transition metal complexes, although careful calibration is needed.