One of the most attractive research in the field of molecular solids is multifunctional molecular magnets coupled with spin, charge and photon, and single-molecular magnet/single-chain magnet toward spintronics. In this chapter, we focus on the molecular magnetism and its related light-induced phase transitions from the viewpoint of Mössbauer spectroscopy. In the Sect. 6.1, the outline of this chapter is described. In the Sect. 6.2, static and dynamic spin crossover phenomena between the high-spin and low-spin states, and the spin frustration system induced by dynamic spin crossover phenomena for [MnIIFeIII(C2O3S)3] complex are described. In the Sect. 6.3, metal complexes showing charge transfer phase transitions such as Prussian blue analogous complexes and [FeIIFeIII(C2O2S2)3] complexes and their photo-induced phase transitions are described. In the Sect. 6.4, various kinds of molecular magnets including single molecular chain magnets are described.