Accurate metacognition – cognition about cognition – is critical for effective self-regulation and good performance on various cognitive tasks. For this reason, there is a real danger that metacognitive illusions undermine cognitive performance. In this chapter, we present an overview of metacognitive illusions in the domains of learning, knowledge assessment, and thinking. On a theoretical level, the existence of metacognitive illusions indicates that people cannot directly access their cognition, but infer the state of their mind from cues and heuristics. The cues that produce metacognitive illusions are categorized into faulty beliefs about cognition and invalid non-analytic cues. This chapter discusses real-life consequences of metacognitive illusions and provides evidence-based recommendations for the mending of specific metacognitive illusions and for creating conditions that foster accurate metacognition.