The idea of the workshop on Functional Integration, Theory and Applications, held in Louvain-la-Neuve from November 6 to 9 1979, was to put in close and informal contact, during a few days, active workers in the field.There is no doubt now that functional integration is a tool that is being applied in all branches of modern physics. Since the earlier works of Dirac and Feynman enormous progress has been made, but unfortunately we lack still a unifying and rigorous mathematical framework to account for all the situations in which one is interested. We are then in presence of a rapidly changing field in which new achievements, proposals, and points of view are the normal pattern.Considering this state of affairs we have decided to order the articles starting from the more fundamental and ambitious from the point of view of mathematical rigour, followed by articles in which the main interest is the application to concrete physical situations. It is obvious that this ordering should not be taken too seriously since in many cases there will be an interplay of both objects.