People are often presumed to be vulnerable to a desirability bias, namely, a tendency to be overoptimistic about a future outcome as a result of their preferences or desires for that outcome. In this article, this form of wishful thinking is distinguished from the more general concepts of motivated reasoning and overoptimism, and the evidence for this bias is reviewed. The authors argue that despite the prevalence of the idea that desires bias optimism, the empirical evidence regarding this possibility is limited. The potential for desires to depress rather than enhance optimism is discussed, and the authors advocate for greater research attention to mediators of both types of effects. Nine possible mediational accounts are described, and critical issues for future research on the desirability bias are discussed.