Weakness of will, the phenomenon of acting contrary to one’s own better judgment, has remained a prominent discussion topic of philosophy. The history of this discussion in ancient, medieval, and modern times has been outlined in many studies. Weakness of Will in Renaissance and Reformation Thought is, however, the first book to cover the fascinating source materials on weakness of will between 1350 and 1650. In addition to considering the work of a broad range of Renaissance authors (including Petrarch, Donato Acciaiuoli, John Mair, and Francesco Piccolomini), Risto Saarinen explores the theologically coloured debates of the Reformation period, such as those provided by Martin Luther, Philip Melanchthon, John Calvin, and Lambert Daneau. He goes on to discuss the impact of these authors on prominent figures of early modernity, including Shakespeare, Descartes, Spinoza, and Leibniz. While most of the historical research on weakness of will has focused on the reception history of Aristotle’s Nicomachean Ethics, Saarinen pays attention to the Platonic and Stoic discussions and their revival during the Renaissance and the Reformation. He also shows the ways in which Augustine’s discussion of the divided will is intertwined with the Christian reception of ancient Greek ethics, and argues that the theological underpinnings of early modern authors do not rule out weakness of will, but transform the philosophical discussion and lead it towards new solutions.