Prefactual thoughts typically take the form of implied or explicit if–then statements that represent mental simulations of alternatives to what is expected to occur in the future. The authors propose that the multiplicative combination of "if likelihood" (the degree to which the antecedent condition of the prefactual is perceived to be likely) and "then likelihood" (the perceived conditional likelihood of the outcome of the prefactual, given the antecedent condition) determine the influence of prefactuals. This construct, termed prefactual potency, is a reliable predictor of the degree of influence of prefactual thinking on judgments of anticipated negative affect. Through three experiments, the authors demonstrate the predictive power of this construct and show that it plays a causal role in determining the strength of the effects of prefactual thought. Implications of prefactual potency as a central factor of prefactual influence are discussed.