When students monitor the effectiveness of their learning and accuracy of their memories, the presence or absence of specific content knowledge is not the only information that guides their evaluations. Equally important are the metacognitive experiences, subjective feelings, and epistemological beliefs that inform and accompany learning and remembering and guide achievement-related behavior. Students use a variety of cues (e.g., Koriat "Journal of Experimental Psychology: General," 126, 349-370, 1997), including experiences of and beliefs about processing fluency to determine confidence in their knowledge. This article addresses why some illusions of knowing that arise while learning and remembering are so pervasive. We draw on converging research from social and cognitive psychology to discuss the allure of processing fluency and influence of metacognitive beliefs on assessments of confidence. We include a discussion of research on the interaction of naïve theories of intelligence with perceptions of processing fluency. Finally, we provide a number of suggestions to mitigate mistakes of confidence.