This book motivates and systematically develops the view that knowledge should be at the centre of our theory of practical rationality. "Only act on what you know!"-the book offers the first comprehensive defence of this slogan in the form of a knowledge-based decision theory. The proposed view is shown to be the most straightforward explanation of emerging bodies of evidence for a link between knowledge and rational action. The book provides novel solutions to well-known challenges that invoke high-stakes scenarios, seemingly rational actions based on reasonable but false beliefs, and decision-making under risk and uncertainty. The emerging view brings together traditional and formal approaches to epistemology and the study of practical rationality. The resulting knowledge-based decision theory is embedded in a rich normative landscape, encompassing innovative accounts of excuses, rational acceptance and the simplification of reasoning. Knowledge and Rational Action will be of great interest to researchers and graduate students interested in epistemology, practical rationality and decision theory.