Strained layer epitaxy, a technique used in the growth of single-crystal semiconductor heterostructures in which ultrathin layers of semiconductors with differing lattice parameters are grown on top of each other, is well suited for the production of high-speed electronic devices. The lattice mismatch causes the accumulation of elastic strain energy which is reduced by the inclusion of defects, such as misfit dislocations. The direct and real-time study of the formation and propagation of these defects using an electron microscope helps us to understand the fundamental processes governing relaxation in heterostructures and is the topic of this review.