This book is concerned with hypothetical thinking, which involves the imagination of possibilities and the simulation of their consequences. It is evident that hypothesis-testing behaviour must be of direct relevance to this enterprise. However, hypothesis-testing tasks differ from others considered in this book (for example, decision making) in that evaluation of imagined possibilities does not rest simply on the ability to simulate mentally their characteristics and consequences, although this may be involved in generating predictions. Hypothesis testing also involves observation of and frequently experimentation with the actual world, gathering evidence relevant to the hypothesis under consideration. As a result, hypotheses may be supported or refuted, and retained, revised or abandoned by the hypothesis tester. What I will attempt to demonstrate in this chapter is that this interactive process conforms with the same principles that describe other kinds of hypothetical thinking. In particular, hypothesis-testing behaviour is subject to the fundamental analytic bias (Table 1.2) in that there is a tendency to consider just one hypothesis at a time (singularity principle) and to retain it unless evidence is discovered that provides a strong reason to give it up (satisficing principle).