Spin-crossover (SCO) complexes are an ongoing challenge to quantum chemistry due to the delicate balance of their electronic and entropic contributions to the adiabatic enthalpy difference between the high- and low-spin states. This challenge has fuelled an improvement in the existing quantum chemical methods and the development of new ones and will continue to do so. The progress in electronic structure calculations performed on SCO complexes in recent years has made quantum chemical methods valuable tools that may aid the design of new SCO compounds with desirable features. Post-Hartree–Fock ab initio methods can be used to calculate the adiabatic energy difference between high- and low-spin states with satisfactory accuracy but are currently limited to model systems or smaller molecular SCO complexes. The results obtained by these methods serve as references for other electronic structure calculations that may also be applied to larger systems. The methods of choice for the calculation of geometries and molecular vibrations of isolated SCO complexes and of crystalline compounds are based on density functional theory (DFT). Recent hybrid functionals can be used to calculate the adiabatic energy difference to an accuracy that is in some cases close to that of ab initio calculations, although no unique functional has been identified up to now that is superior to other functionals in all cases. DFT methods can now also be applied to crystalline systems and allow intermolecular effects to be investigated that are important for understanding the cooperativity of spin transitions.