This chapter comprehensively addressed nanomaterials and their mechanical characteristics, deformation processes, and uses in many different disciplines. With a particular focus on grain boundaries, dislocation dynamics, and grain size effects, the chapter examines new studies about how nanoscale features influence material behavior. Modern experimental characterization of nanomaterials and molecular dynamics simulations—has made it feasible to understand atomic-level mechanisms controlling nanomaterials’ performance. With nanocrystalline microstructures, the chapter emphasizes significant developments in knowledge of fatigue resistance, fracture toughness, and strengthening processes in metals, ceramics, and composites. One of the main goals is to improve mechanical properties by using interfaces, twin barriers, and networks of nanoparticles. It also addresses the relevance of the acquired results in practical applications spanning structural materials to biomedical devices. We address how multiscale modeling approaches and computational methodologies forecast and maximize the behavior of nanomaterials. Integration of experimental data with several theoretical models generated discoveries in the creation of materials with tailored-made qualities for specific purposes. This thorough analysis confirms that, compared to traditional materials, nanoscale engineering creates opportunities for the creation of next-generation materials with improved strength, ductility, and durability.