This book explores in depth the role of dislocations in materials, covering their historical discovery, their impact on mechanical properties, and their modeling. After an introduction to the definition and structure of dislocations, the text presents the fundamental theory and mechanisms of dislocation displacement, illustrated by practical examples. Observation techniques such as transmission electron microscopy and X-ray diffraction are described in detail. The book also looks at practical applications of dislocations in superalloys and nanostructured materials, highlighting specific case studies. The management of dislocations to optimize material properties is discussed, offering an overview of current advances and future challenges in the field.