If a just-browsing reader were to open the book I would like them to begin reading the Introduction. The Introduction seeks to explain why the animus concept is so important, but also why our common-sense understanding of the term is insufficient for purposes of constitutional law.The Introduction is short. It begins with a simple (indeed, simplistic) set of three government actions that treat people unequally, and asks, Sesame Street-style, which of them is unlike the other two. The book provides the answer: one of those situations, a city council’s decision to deny a zoning variance to a proposed group home for intellectually disabled persons based on constituents’ dislike of those persons, is based in animus. Indeed, this situation reflects the facts of the City of Cleburne case that has played an important role in the development of animus doctrine.Identifying this situation as one where the government action was motivated by animus provides an opening to the idea of animus. In particular, it will be intuitively obvious to many readers that the city’s action seems unconstitutional. In turn, that insight opens the door to realizing that the framers of the Constitution had a similar attitude. Framers such as Madison understood that groups of persons (what Madison referred to as “factions”) may hijack government for their own private ends—that is, to accomplish goals unrelated to the public good. Such conduct distantly, but distinctly, echoes the idea of animus. After all, as the book explains, animus essentially amounts to a government action to disadvantage a group for no legitimate reason.With animus thus understood as an instinctive constitutional wrong, and as one that American constitutional law has always condemned, the question then becomes what the concept means. The Introduction explains that the intuitive understanding of animus as subjective bad intent doesn’t easily fit the context of government action. Government action is often institutional—the action of a legislature or an administrative agency, for example. Thus, it is difficult to translate into legal doctrine our intuitional understanding of animus as subjective bad intent. The book promises to provide an understanding of animus that is both workable as legal doctrine and faithful to the intuition readers had when they confronted the example of animus on the first page.In sum, then, the Introduction identifies animus as an issue that Americans should care about, explains (briefly) why the issue is complex, and promises to create a doctrine that courts can use to guard against it. As such, I hope it hooks readers to come along on the book’s exploration of animus.I have several hopes for this book. First, like most legal scholars, I hope to influence judges. Animus doctrine is quite important, but it’s seriously under-theorized. I hope that the book convinces judges that the Supreme Court’s previous animus cases have implicitly created a coherent animus doctrine that judges can use when deciding cases. The book attempts to reveal and illustrate that doctrine, and to present it in a way that judges can employ when deciding actual cases.I also hope this book triggers more theoretical discussion among legal scholars about the future of equal protection law. Equal protection law today is adrift. As I said earlier, the Court has largely abandoned the political process theory approach. Nothing has replaced it. Animus doctrine can’t fill that hole completely. However, it can provide part of the solution. That part is an important one: the rise of xenophobia and cultural conflict (for example about sexual orientation and transgender identity) has raised the specter of government action motivated by simple dislike of particular persons—the core of the animus idea. Thus, a well-crafted animus doctrine can play an important role in the development of an approach to equal protection that responds to the issues prevalent in twenty-first century America.Finally, and most generally, I hope the book triggers thought and discussion among legal experts, students, and educated laypersons, about what our constitution should mean. In writing this book I intentionally avoided overly-technical jargon and fine analytical distinctions, in favor of broad-brush analysis. Such technicalities have their place. But there is an important place for informed discussions between citizens about what their constitution means, and what it should mean.Unfortunately, many Americans know little about fundamental constitutional concepts such as equal protection. This book attempts to introduce Americans to an important aspect of that concept, in a sophisticated yet accessible way. The issues to which animus doctrine responds are important ones in modern America. For that reason, Americans should be acquainted with, and feel able to discuss, ways our constitutional system can respond to those issue. One of those ways is through the animus doctrine. I hope this book contributes to Americans’ understandings of that idea, and stimulates critical thinking about it.