The papers appearing in this volume are part of those originally intended for presentation at the conference: Logic Colloquium '80 - European Summer Meeting of the Association for Symbolic Logic (A.S.L.) which was to takeplace in Prague, August 24·30, 1980, principally under the auspices of the Czech Academy of Sciences. There were 36 invited speakers from Western and Eastern Europe, Israel, the U.S., and the U.S.S.R. The local organizingcommittee cabled participants on July 15, 1980 to inform them that the meeting was cancelled for technical reasons; a subsequent communication stated that the cancellation was due to unforeseen circumstances lying beyond the controlof the organizing committee. The unexpected cancellation of the Prague meeting was greatly regretted, since so much care, time, and energy had been given to its advance preparation by the local organizing committee as well as by representatives of the A.S.L.and its European Committee. The late date on which cancellation took place required drastic changes of plans by speakers and participants. Last-minute efforts to reschedule the meeting elsewhere in Europe could not be realized.