Hypothetical thinking involves imagining possibilities and mentally exploring their consequences. This chapter overviews a contemporary, integrative account of such thinking in the form of Jonathan Evans’s hypothetical thinking theory. This default-interventionist, dual–process theory operates according to three principles: relevance, singularity, and satisficing. To illustrate the explanatory strength of the theory a range of empirical evidence is considered that has arisen from extensive research on hypothesis testing, which involves individuals generating and evaluating hypotheses as they attempt to derive a more general understanding of information. The chapter shows how key findings from hypothesis-testing research undertaken in both laboratory and real-world studies (e.g. in domains such as scientific reasoning) are readily explained by the principles embedded in hypothetical thinking theory. The chapter additionally points to important new directions for future research on hypothetical thinking, including the need for: (1) further studies of real-world hypothesis testing in collaborative contexts, including ones outside of the domain of scientific reasoning; (2) increased neuroscientific analysis of the brain systems underpinning hypothetical thinking so as to inform theoretical developments; and (3) systematic individual-differences investigations to explore the likely association between people’s capacity to think creatively and their ability to engage in effective hypothetical thinking.