This is a preface to the book "Spin-Crossover Materials: Properties and Applications" edited by M.A. Halcrow. The preface discusses the spin-crossover process, which involves the rearrangement of electrons in a metal ion from a high-spin to a low-spin state. This phenomenon is particularly prevalent in iron chemistry and affects the physical properties of solid materials, including magnetic moment, color, dielectric constant, and electrical resistance. Spin-crossover materials exhibit hysteresis, indicating structural phase changes during transitions, and can function as bistable switches. Several practical applications of spin-crossover materials have been demonstrated, including display and memory devices, electrical and electroluminescent devices, and temperature-sensitive MRI contrast agents. Switchable liquid crystals, nanoparticles, and thin films have also been developed. Most applications have used materials with spin-transitions showing thermal hysteresis of 30-50 K. The development of new spin-crossover materials with useful properties remains a challenge in crystal engineering. The preface notes that the "bible" in the field is a three-book set published in 2004. This book complements that work by focusing on recent developments in spin-crossover research. The first four chapters provide an overview of spin-crossover research and detailed surveys of various spin-crossover complexes. The next chapters discuss alternative types of spin-state transitions, including charge-transfer-induced spin-transitions and spin-pairing between organic radical centers. The following chapters cover the application of spin-crossover compounds in multifunctional materials and nanotechnology. The final chapter discusses the importance of pressure-induced spin-crossover to geology. The book also honors Andrés Goeta, who passed away before the book was completed. The preface concludes with a dedication to Goeta's memory.This is a preface to the book "Spin-Crossover Materials: Properties and Applications" edited by M.A. Halcrow. The preface discusses the spin-crossover process, which involves the rearrangement of electrons in a metal ion from a high-spin to a low-spin state. This phenomenon is particularly prevalent in iron chemistry and affects the physical properties of solid materials, including magnetic moment, color, dielectric constant, and electrical resistance. Spin-crossover materials exhibit hysteresis, indicating structural phase changes during transitions, and can function as bistable switches. Several practical applications of spin-crossover materials have been demonstrated, including display and memory devices, electrical and electroluminescent devices, and temperature-sensitive MRI contrast agents. Switchable liquid crystals, nanoparticles, and thin films have also been developed. Most applications have used materials with spin-transitions showing thermal hysteresis of 30-50 K. The development of new spin-crossover materials with useful properties remains a challenge in crystal engineering. The preface notes that the "bible" in the field is a three-book set published in 2004. This book complements that work by focusing on recent developments in spin-crossover research. The first four chapters provide an overview of spin-crossover research and detailed surveys of various spin-crossover complexes. The next chapters discuss alternative types of spin-state transitions, including charge-transfer-induced spin-transitions and spin-pairing between organic radical centers. The following chapters cover the application of spin-crossover compounds in multifunctional materials and nanotechnology. The final chapter discusses the importance of pressure-induced spin-crossover to geology. The book also honors Andrés Goeta, who passed away before the book was completed. The preface concludes with a dedication to Goeta's memory.