We make a decision every time that we do something when we could have done something else, but most of these decisions are habitual and require little thought. ‘Decision making’ explains that hypothetical thinking is essential to deliberative decision making. The psychology of decision making has been much influenced by the study of economics, which provides a theory of rational decision making. This theory requires that people project future consequences of their choices, estimating probabilities and values and generally choosing the options with the best (mathematically) expected value. Against this we have a wealth of psychological evidence that both decision making and probability judgement are subject to a whole host of cognitive biases.