Magnetic contributions to heat capacities of molecule-based magnets and assembled-metal complexes are explained. Although magnetic moments of both spin and electron-orbitals are involved in the magnetic phenomena occurring in the solid state, primal role is usually played by spins. Since spin is a physical quantity definitely defined by quantum mechanics, magnetic heat capacities experimentally determined are often used as a diagnostic tool to check the validity of the theoretical treatments proposed in the field of solid state physics. Discussed in this paper are (1) low-dimensionality and dimensional-crossover encountered in molecule-based magnets and assembled-metal complexes, (2) heat capacity of single-molecule-magnet under magnetic field, (3) magnetic phase transitions due to order-disorder mechanism of spin orientation, (4) spin-crossover phenomena in which the electronic state itself is altered by temperature, (5) interplay between charge-transfer and spin-state conversion, and so on.