This book offers a novel account of Aquinas's theory of the human act. It argues that Aquinas takes a human act to be a composite of two power-exercises, where one relates to the other as form to matter. The formal component is an act of the will, and the material component is a power-exercise caused by the will, which Aquinas refers to as the 'commanded act.' The book also argues that Aquinas conceptualizes the act of free choice as a hylomorphic composite: it is, materially, an act of the will, but it inherits a form from reason. As the book aims to show, the core idea of Aquinas's hylomorphic action theory is that the exercise of one power can structure the exercise of another power, and this provides a helpful way to think of the presence of cognition in conation and of intention in bodily movement.