This book originates in the French classic "Principes de Tectonique" (Masson, 1983), written by professor Adolphe Nicolas, and the more recent "Principes de Tectonique" by J.L. Bouchez and A. Nicolas (De Boeck, 2018). This English edition is an up-to-date and augmented version that keeps the concise and rigorous writing of its inspiring predecessors. It is largely based on laboratory and field experience of both authors, with a focus towards hard rocks and magmatic rocks from both the continental crust worldwide and the mantle, principally from the Oman ophiolites. The book includes more than 250 illustrations, most of them original. In addition to classic geological subjects, the book includes elements such as plastic deformation of ice, quartz and olivine, fabric acquisition in rocks and magmas, measurement and orientation of stress, together with basic background information on neotectonics, geophysics and other practical tools such as magnetic fabrics not commonly treated in geological books. Since the targeted readers are present day young students, a few exercises of structural geology are included to improve their abilities. This book aims principally at students of Geology, at both the undergraduate and graduate levels. However, due to its numerous illustrations and rather concise writing, anyone interested in rock deformation and/or tectonics will find key answers in this book.