The introductory chapter of this book offers a brief account of the relation between action theory and moral philosophy in Aquinas. It argues that Aquinas has a descriptive, metaphysical account of the human act, one that investigates the human act’s ontology as well as its aetiology, that is, respectively, what the human act is and how it is explained. This account brackets normative considerations about what acts are morally good or bad. The introduction specifies that the book deals with this descriptive theory, and it also motivates the book’s main textual focus, which is on one aspect of Aquinas’s Prima secundae discussion of the human act, namely, the phase leading from choice (electio) to the actual performance of the human act. Finally, the introduction states the main thesis of the book, which is that both choice and the human act that it explains are hylomorphically structured, for Aquinas. Choice is a composite of a volition and a preferential intentional structure inherited from a previous judgment, and the human act is a composite of a volition and a power-exercise caused by volition, such as a bodily movement.