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 industry were invited to review their fields of interest. The expert presentations of these scientists were complemented by contributed papers and poster contributions, 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.