This book proposes a theory of Media Acumen, a developmental framework that bridges theories about how adults navigate information credibility in contemporary media environments.Integrating established research traditions with cognitive science research on implicit associations, pattern recognition, and misconception persistence, the book explores alternatives to existing media literacy approaches. The author proposes testable predictions for empirical research, provides operationalizable constructs for behavioral assessment, and suggests design principles for informal learning interventions that work with how human cognition actually functions. Examining contemporary crises, the book demonstrates how vulnerable adults become when pattern recognition systems calibrated for 20th-century media encounter today's sophisticated manipulation tactics and misinformation.By bringing together theory with practical intervention strategies, this book offers researchers, educators, policymakers, and media professionals a roadmap for building information literacy at population scale in the age of AI and synthetic media. It will interest academic researchers and graduate students in fields grappling with information credibility challenges, most notably media literacy, education, communication studies, and developmental and cognitive psychology.