A variety of formalisms have been developed to address suchaspects of handling imperfect knowledge as uncertainty,vagueness, imprecision, incompleteness, and partialinconsistency. Some of the most familiar approaches in thisresearch field are nonmonotonic logics, modal logics,probability theory (Bayesian and non-Bayesian), belieffunction theory, and fuzzy sets and possibility theory...ESPRIT Basic Research Action 3085, entitled DefeasibleReasoning and Uncertainty Management Systems (DRUMS), aimsto contribute to the elucidation of similarities anddifferences between these formalisms. It consists of 11active European research groups. The European Conference onSymbolic and Quantitative Approaches to Uncertainty (ESQAU)provides a forum for these groups to meet and discuss theirscientific results...This volume contains 42 contributions accepted for the ESQAUmeeting held in October 1991 in Marseille, together with 12articles presenting the activities of the DRUMS groups andtwo invited presentations.