Among the many practical failures that threaten us, weakness of will or akrasia is often considered to be a paradigm of irrationality. The eleven new essays in this collection, written by an excellent international team of philosophers, some well-established, some younger scholars, give a rich overview of the current debate over weakness of will and practical irrationality more generally. Issues covered include classical questions such as the distinction between weakness and compulsion, the connection between evaluative judgement and motivation, the role of emotions in akrasia, rational agency, and the existence of the will. The also include new topics, such as group akrasia, strength of will, the nature of correct choice, the structure of decision theory, the temporality of prudential reasons, and emotional rationality. Because these questions cut across philosophy of mind and ethics, the collection will be essential reading for scholars, postgraduates, and upper-level undergraduates in both these fields.