Although overweight is undesirable from both public and personal health perspectives, the focus of this paper is on exploring the nature of fat prejudice within a social cognition framework. Fat prejudice refers to the tendency to form judgments about people on the basis of excessive body weight. Body size has been described as one of the few personal attributes considered an acceptable target of prejudice, despite the fact that targets of fat prejudice experience significant psychological distress. Fat prejudice is likely to become an increasingly common psychosocial problem in light of the obesity epidemic that is currently affecting many countries. The current paper reviews findings from nineteen experimental studies of implicit anti-fat attitudes; these studies have used either the implicit association test or the affective priming task. The empirical data highlight that implicit anti-fat attitudes are widely held and relatively universal. Robust implicit anti-fat bias is evident among many groups including university students, members of the general public, health professionals, and among those who are themselves overweight or obese. The current data suggest that, similar to findings with other attitudinal objects, the relationship between implicit and explicit measures of anti-fat attitudes is complex. The possibility of changing implicit anti-fat attitudes, either by modifying the underlying associative structures or by altering the pattern of activation, is discussed. Avenues for future research are offered, keeping in mind the challenge of formulating appropriate public health messages whilst also challenging weight bias, and promoting acceptance of diversity in body size.