This book contains papers that were presented at the International Conference on Polycrystalline Semiconductors - Grain Boundaries, Dislocations and Het­ erointerfaces - (POLYSE '90), which was held in Schwabisch Hall, FRG, from July 30 to August 3, 1990. This conference was a satellite conference of the 20th International Conference on the Physics of Semiconductors. POLYSE '90, like its predecessor POLYSE '88, brought together scientists from research in­ stitutions and industrial laboratories with a view to bridging the gap between fundamental and technological aspects of polycrystalline semiconductors. With this aim, a total of 14 recognized scientists from universities and in­ dustry were invited to review their fields of interest. The expert presentations of these scientists were complemented by contributed papers and poster con­ tributions, the authors of which were additionally allowed four minutes for an oral summary. This combination of different types of presentation led to very lively and stimulating discussions.