The essays appearing in these two volumes are based on Keynote (Vol. 1) and State-of-the-Art (Vol. 2) Lectures delivered at the XXVth International Congress of Psychology, in Brussels, July 1992. The Brussels Congress was the latest in a series of conferences which are organized at regular intervals under the auspices of the International Union of Psychological Science (IUPsyS), the main international organization in the field of Scientific Psychology. The first of those meetings took place in Paris in 1889. An important function of the International Congresses is to promote communication between different specializations in Psychology. Speakers were therefore asked to present lectures and discussions in their own fields of study, in a way that would be accessible to fellow psychologists active in other fields. State-of-the-Art lecturers were specifically asked to prepare a tutorial review on a topic which, in the view of the Program Committee, had recently given rise to particularly important developments. These contributions are included in Volume Two. Keynote lecturers were left free to address whatever subject they felt was of greatest interest. The chapters in Volume 1 are preceded by the Presidential Address by Mark R. Rosenzweig.