This Element provides a comprehensive introduction to contemporary theories of mental content. After clarifying central concepts and identifying the questions that dominate the current debate, it presents and discusses the principal accounts of the nature of mental content (or mental representation), which include causal, informational, teleological and structuralist approaches, alongside the phenomenal intentionality approach and the intentional stance theory. Additionally, it examines anti-representationalist accounts which question either the existence or the explanatory relevance of mental content. Finally, the Element concludes by considering some recent developments in the debate about mental content, specifically the "explanatory turn" and its implications for questions about representations in basic cognitive systems and the representational character of current empirical theories of cognition.