As electron microscopy is concerned with objects of more complex crystallographic structures using instruments of higher acceleration voltages, one more and more encounters features of many-beam electron diffraction. This expands the methodical varieties, particularly of imaging crystal defects by diffraction contrast. The present work is to serve as a theoretical and practical guide for exploiting these possibilities. It comprehensively reviews the required dynamical and kinematical diffraction theory thus closing some gaps, e. g. by systematically comparing "ordinary" and "modified" Bloch waves, or by denoting conditions of a first-order critical voltage effect. It is shown how to classify and simplify many-beam situations and imaging conditions. Besides typical examples, data files are given in which extinction distances up to 1 MV accelerating voltage are included.