Observation of the deformation of thin aluminum specimens inside the transmission electron microscope has revealed the intimate connection between deformation and the nucleation and motion of dislocations. Nucleation, slip propagation, and the progress of deformation to the point of rupture have been observed visually on the fluorescent screen and photographed by both ``still'' and motion pucture techniques. In this way even the detailed predictions of the dislocation theory of deformation have been confirmed, and the new phenomena of grain boundaries acting as ``donors'' or ``acceptors'' of dislocations have been discovered.