The models of dislocation generation during plastic deformation are reviewed. The different types of dislocation generation, i.e. localized Frank?x2013;Read sources and multiplication by the double-cross slip mechanism, are observed in ceramic single crystals, semiconductors, metals, intermetallics and quasicrystals during in situ deformation in a high-voltage electron microscope. The results are discussed with respect to the ability of cross slip in different materials.