Decision-makers in business and economics face a staggering array of problems. For example, managers of growing firms have to decide when to expand their business, governments have to decide whether to undertake large infrastructure investments, and managers of oil firms must decide how rapidly to deplete their reserves. While these problems seem quite diverse, they all share many important features. In each case, the decision-maker must choose when to take a particular action that will be potentially impossible to reverse, and the consequences of taking (or not taking) that action are uncertain.