A common stereotype about older adults is that they are forgetful, incompetent, and cognitively inferior to younger adults (Hummert, Garstka, Shaner, & Strahm, 1994). Older adults tend to underperform on tests of cognitive abilities when they are reminded of these negative aging stereotypes, presumably due to the fear of confirming these stereotypes through their behavior. Age differences in ability disappear, however, when the stereotypes are not salient in the testing situation. This phenomenon is called stereotype threat, and its effects have been observed in numerous stereotyped groups. This entry discusses how and why stereotypes lead to threat effects on performance, and evidence of age differences in the underlying mechanisms. It concludes with a discussion of current and future directions in age stereotype research.