We talk about irrationality when behaviour defies explanation or prediction, when decisions are driven by emotions or instinct rather than by reflection, when reasoning fails to conform to basic principles of logic and probability, and when beliefs lack coherence or empirical support. Depending on the context, agents exhibiting irrational behaviour may be described as foolish, ignorant, unwise or even insane. In this clear and engaging introduction to current debates on irrationality, Lisa Bortolotti presents the many facets of the concept and offers an original account of the importance of judgements of irrationality as value judgements. The book examines the standards against which we measure human behaviour, and reviews the often serious implications of judgements of irrationality for ethics and policy. Bortolotti argues that we should adopt a more critical stance towards accepted standards of rationality in the light of the often surprising outcomes of philosophical inquiry and cognitive science research into decision making. Irrationality is an accessible guide to the concept and will be essential reading for students and scholars interested in the limitations of human cognition and human agency.