The topic of habitus is one of Thomas Aquinas’s greatest contributions to moral theology, but it has been generally neglected in theological scholarship until now. Habits and Holiness is the first work in English to explore Aquinas’s rich theology of habit in all of its grandeur and depth. Habits and Holiness shows that most facets of human life and behavior are greatly influenced by habits, which Thomas appraises as an analogous concept that is much broader than previous scholarship has recognized. Habits and Holiness accomplishes three tasks. First, it gives a complete and coherent account of Aquinas’s account of habitus. Most accounts of Aquinas’s view of habitus focus almost exclusively on “Treatise on Habits” in the Summa Theologiae I-II, qq. 49-54, and speak of habitus in reference to the virtues. However, Aquinas speaks of habitus in many other places, especially his commentaries on Aristotle’s works and his commentaries on Sacred Scripture. Aquinas employs the concept of habitus to explain a wide variety of human inclinations, such as instincts, personal and societal custom, acquired skills and virtues, original sin, grace, infused virtues, and Gifts of the Holy Spirit. Second, this book indicates how biological psychology illuminates and enriches Aquinas’s account of habit, and vice versa. Finally, Habits and Holiness provides readers with a framework for interpreting and utilizing the vast amount of practical habit literature that exists: it offers a practical analysis of habit development found in Aquinas’s works and those of empirical studies. By seeing habits in general as a prism for understanding human action and its influences, Habits and Holiness provides a unique and appealing synthesis of Thomistic virtue theory, the contemporary science of habits, and best practices for eliminating bad habits and living good habits.