Engaging in metacognition (evaluating, controlling, and representing cognitive states) is paramount for efficient behaviour. In this Review, we examine different types of cognitive architectures that might be at play when people provide metacognitive judgements in the domains of memory and perception. Building upon this conceptual framework, we review evidence supporting and challenging domain-general metacognition. We also discuss commonalities in metacognition across domains, focusing on the influence of decisional processes on metacognitive judgements. We emphasize the challenges of isolating metacognitive processes and how these challenges influence conclusions regarding the domain generality of metacognition, including in clinical conditions that are hypothesized to have metacognitive impairments. Finally, we give an overview of ‘adecisional’ metacognition: evaluations made outside the context of a decisional process. We find no evidence for a strong form of domain generality but outline how such an architecture could be identified in future research.