The preface to the book *Spin-Crossover Materials: Properties and Applications* by Malcolm Halcrow provides an overview of the field of spin-crossover materials, which involve the rearrangement of electrons in metal ions between high-spin and low-spin states. This phenomenon significantly impacts the physical properties of materials, including magnetic moment, color, dielectric constant, and electrical resistance. Spin-crossover materials have practical applications in display and memory devices, electrical and electroluminescent devices, and MRI contrast agents. The book aims to complement earlier work by providing detailed surveys of mononuclear, polynuclear, and polymeric spin-crossover complexes discovered since 2004, as well as alternative types of spin-state transitions and their applications. It also covers advances in physical and theoretical methods for studying these materials and their importance in geology, particularly in understanding the Earth's mantle. The preface concludes with a dedication to Andrés Goeta, a pioneer in the study of excited spin-states in spin-transition materials, who passed away suddenly.The preface to the book *Spin-Crossover Materials: Properties and Applications* by Malcolm Halcrow provides an overview of the field of spin-crossover materials, which involve the rearrangement of electrons in metal ions between high-spin and low-spin states. This phenomenon significantly impacts the physical properties of materials, including magnetic moment, color, dielectric constant, and electrical resistance. Spin-crossover materials have practical applications in display and memory devices, electrical and electroluminescent devices, and MRI contrast agents. The book aims to complement earlier work by providing detailed surveys of mononuclear, polynuclear, and polymeric spin-crossover complexes discovered since 2004, as well as alternative types of spin-state transitions and their applications. It also covers advances in physical and theoretical methods for studying these materials and their importance in geology, particularly in understanding the Earth's mantle. The preface concludes with a dedication to Andrés Goeta, a pioneer in the study of excited spin-states in spin-transition materials, who passed away suddenly.