The first observations of dislocations by TEM were published in 1956. Since then the technique has been developed into an indispensable tool for the materials scientist, not only for the characterization of the extended defect structure of materials, but also for the elucidation of the mechanisms controlling their properties. To mark the 50th anniversary of the original work, this Special Issue consists of reprints of the three original papers and some 22 current state-of-the-art articles, which provide a snapshot of the (...) tremendous power and versatility of the technique at the present time. It is also hoped that this collection of papers from different areas of application and including different techniques might stimulate some cross-fertilization. In this introduction, we comment briefly on the context in which the original work was performed, trace some of the important developments that have taken place in the intervening period and identify some of the outstanding problems. (shrink)