The use of industrial chemicals to eradicate foliage in the name of economic, scientific, and military progress was a cornerstone of post–World War II US policy. Needless to say, their use also provoked significant controversy within the United States and across the world. It is that controversy that Amy H. Hay explores in her new book. The Defoliation of America is not only a history of Agent Orange chemicals (phenoxy herbicides) but is, more importantly, an examination of how diverse networks of grassroots activists questioned state and scientific authority. The book offers new and interesting insights into the history of how “ordinary citizens” became activists and the processes through which they challenged the use of phenoxy herbicides in agriculture, land management, and warfare.The book is divided into three sections each containing three chapters, as well as an introduction and conclusion. The first section explores the origins of phenoxy herbicides and “the quickening of conscience” around their use in the United States, Europe, and Vietnam. The second section offers case studies of phenoxy spraying and citizen response in the US West, while the third considers the toxic legacies of phenoxy herbicides, including their use in the War on Drugs. The Defoliation of America is an expansive book that links disparate networks of activism together in a coherent way and does an excellent job of interrogating the contested nature of knowledge making around phenoxy herbicides. I look forward to teaching it in my upper division science, technology, and society classes.I was particularly intrigued by Hay's eloquent descriptions of how people became activists. For some, it was reading about or participating in Operation Ranch Hand, while for others it was the spraying of their bodies or their land. This fascinated me because it highlighted something that Hay does so well, situating scientific authority and processes of knowledge making within larger structures of power. Many of the protest networks that Hay described were successful in contesting the scientific and state authority to determine the nature of proof and harm, but it is also true that many of the successes of these “anti-toxics” campaigns were little more than a way for chemical companies to buy time: US policies gave chemical companies the benefit of the doubt when it came to any potential harm. The protests of “ordinary citizens” shaped the discourse around phenoxy herbicides use in war and agriculture, but so too did the more powerful chemical companies—most often by sowing doubts of chemical harm deep within our policies and institutions.My reading of Hay's book left me with a plethora of negative emotions. I was angry for the countless Vietnamese harmed and still yet to be harmed in the name of the Cold War. I felt sad for the US soldiers who had survived war only to die from so-called manufacturing mistakes or improper use. I empathized with the people catalyzed to be activists and the struggles they faced. I was again disheartened by yet another story of agricultural laborers living with outsized exposure. Perhaps that is why I found the story of Ida Honorof in chapter 5 so compelling. For all of the stories of contested knowledge making that Hay describes, the one characteristic that permeated them all was the lack of significant accountability. Ida Honorof, first and foremost, was about accountability. She advocated for incarceration of those who had exposed the public to widespread harm. I think her observations around lack of accountability still ring true today. Thus, the main question I could not shake after reading the Defoliation of America is what accountability would actually look like. I think it would look like what Ida said it would, at least in part.The Defoliation of America is an original and fascinating look at the contested history of knowledge making around phenoxy herbicides. But I think one of its most lasting contributions will come from the larger questions it asks—important questions that have resonance for who we want to be today and what our future agriculture looks like. In particular, throughout the book, Hay struggles with the why of chemical herbicides. I think that this is key. Agricultural historians have written so well on the what and the how of pesticides. Perhaps we need to take Hay's lead and ask the why question more.