Magnetic properties in molecular magnets originate from interactions within well defined structural fragments. The latter might build up a molecular crystal, but they may also remain stable in a solution or when deposited on a surface. These systems have a typical size of up to several hundreds of atoms and exhibit a number of interesting physical properties, such as quantum tunneling of magnetization. Molecular magnets are possible candidates for applications in data storage, quantum computing or medicine. First-principles theory descriptions of molecular magnets include quantum-chemistry simulations, as well as density-functional calculations. Both approaches have their advantages and preferential domains of applicability and both constitute technically very demanding tasks in coping with large, non-symmetric and very unevenly packed molecular systems. An overview of the general situation in the field will be given, particularly regarding the relation of calculated electronic properties with experimental observations by different techniques. Moreover some examples of the success and failures of density-functional calculations for several selected molecular magnet systems will be shown.