This volume contains the revised papers of an international symposium on research on fallacies, biases, and the development of decision behavior under uncertainty. The papers are organized in five main sections. The Introduction outlines the conceptual framework and how three of the sections-Cognitive Decision Research, Social Interaction, and Development and Epistemology-are interrelated and also how new fields, such as research into developmental questions, can be productively integrated. In the fifth section Comments are collected, which evaluate the impact of the contributions on decision research itself, and also on cognitive psychology, social psychology, economic theory, ant the discipline of mathematics education.