This book serves both as a text book and a reference guide for Master's and Ph.D. students, focusing on the geochemical principles that govern the formation of hydrothermal ore deposits. It addresses fundamental topics such as mineralogical thermodynamics, major and trace element geochemistry, isotopic compositions of ore and gangue minerals, and the chemistry of hydrothermal fluids. These concepts provide the foundation for understanding the genesis of ore deposits of metals such as copper, molybdenum, zinc, lead, gold, silver, tin, tungsten, and, importantly, rare earth elements. Each chapter is structured to enhance learning through solved numerical problems, self-assessment questions, and case studies drawn from globally significant ore deposits, with particular attention to examples from India. To encourage deeper study, selected references are provided at the end of each chapter and the book concludes with a comprehensive bibliography.