We all feel that our world is fundamentally uncertain. But less unanimity emerges as to what we mean by "uncertainty" and how we ought to adjust. Businessmen tend to be "cautious" and engineers to pick up the "right" decision criteria among the many available in the literature, without having much justification about their respective behaviors. This book tries to sort out the different meanings of uncertainty and to discover their foundations. It shows that uncertainty can be represented using various tools and mental guidelines. Some decision criteria are then related to each case and assessed. Alternative ways to deal with risk - and risk attitude concepts - are then examined in the above perspective. Behavior under uncertainty emerges from this book as something to base more on inquiry and reflection than on mere intuition.