This is a brief summary of the author’s book, The Mismeasure of Desire: The Science, Theory, and Ethics of Sexual Orientation (1999), the first detailed, systematic, and critical discussion of scientific research on sexual orientation, its theoretical underpinnings, its methodology, its scientific merit, and its legal and ethical significance. The author questions typical assumptions about human sexual behaviors and desires as they relate to a person’s attractions to people of a certain sex or gender. The book is divided into three parts that focus, respectively, on metaphysical, scientific, and ethical questions concerning sexual orientation. In this précis, the author summarizes some of the arguments and conclusions from each part of the book.