Very many structural materials like metals and ceramics are of crystalline nature.Undermostoftheconditions,theyundergoplasticdeformationbefore they fail under load. The plastic deformation is mediated by the gene- tion and motion of one-dimensional crystal defects, the so-called dislocations. Thus, the density and dynamic properties of the dislocations determine the plastic behavior of the respective materials and frequently also their failure. Thedislocationdynamics,that is,the responseofthedislocationsto anext- nal load, is controlled by the interaction between the moving dislocations with the periodic crystal lattice structure, other crystal defects from point defects oversmall clusters to largerprecipitates, with other dislocations fo- ing a microstructure through which the mobile dislocations have to move, and ?nally the grain and phase structure of the material. Considering these di?erent interactions, the dislocation motion turns out to be quite a complex process. In situ straining experiments in a transmission electron microscope are a powerful means for studying the microprocesses controlling the dislocation mobility. With this method, dislocations candirectly be observedduring their motionunderload.Thetechniqueadvancedwhenhigh-voltageelectronmic- scopes became commercially available at the end of the 1960s, ?rst in Japan and later on also in other countries. These microscopes enable the transm- sion of thicker specimens, which have properties similar to macroscopic bulk specimens.Besides,they o?ersu?cientroomfor elaboratedeformationstages in the specimen chamber. In the last 30 years, in situ straining experiments were performed on many materials, both in conventional and in high-voltage electron microscopes. The present author and his coworkers performed such experiments over about 30 years, yielding many hours of video recordings of dislocation motion.