Mechanisms of dislocation generation and methods of crystal growth are two historically rich areas of scientific study. These two fields converge in the area of metamorphic epitaxial materials, where the goal is to produce high-performance devices that contain high densities of crystal defects in regions of the engineered material away from the active areas. Metamorphic epitaxy is a form of thin-film growth, where the lattice structure of the layer and substrate are mismatched, and its defining characteristic is that any elastic strain in the overlayer has been relaxed by the deliberate introduction of dislocations at the film–substrate interface. Metamorphic growth enables novel combinations of relaxed single-crystal materials to realize novel functionality and performance in numerous technological areas, including lasers, photovoltaics, transistors, and quantum computing. Many of the devices described in this issue are impossible to realize using the traditional approach of avoiding dislocation generation; instead, they rely on metamorphic epitaxy to attain high performance.