This chapter introduces the concept of metacognition from a cognitive perspective, where it refers to knowledge and mental processes that operate on one’s own cognition. We review different forms of metacognition that involve distinct types of explicit reasoning and automatic processes, as well as various measures and functional benefits. We articulate four conjectures regarding the nature of metacognition in the specific context of the ACT-R cognitive architecture: (1) it involves extracting information about processes in cognitive modules; (2) the information is quantitative and approximate rather than symbolic; (3) the metacognitive information is available in working memory for cognitive processing; and (4) general cognitive processes are sufficient to respond to a situation detected by metacognitive monitoring. We illustrate these principles with examples of past work involving neuro-symbolic models of perception and introspection into declarative models of decision-making. Finally, we situate this approach within the context of theories such as predictive coding and the Common Model of Cognition encompassing other cognitive architectures.