A reader who picked up The Known Citizen and opened it at random might come across social outrage over candid photographs in the late nineteenth century, prompting a spate of “right to privacy” suits brought by women whose images were used without authorization to advertise flour or soap.Another might come across the debates triggered by the new Social Security numbers in the 1930s, and the methods of state documentation and tracking they made possible. If that reader flipped a little further ahead, to the middle of the book, she or he might land on arguments over psychological testing in the schools, social research experiments, contraceptive counseling, and early computer data banks—each of which, in one way or another, raised concerns about individual privacy in postwar American society.A reader who happened instead upon the final chapters would encounter instead the pronounced unease attached to the outflow of personal matters into public venues by the late twentieth century, in the form of early reality television in the 1970s or confessional memoirs in the 1990s. Still a different, and more familiar landscape, of privacy concerns—clouds, data aggregators, retinal scans—would greet the person who turned right to the epilogue.I would hope such browsers would be intrigued rather than bewildered by this kaleidoscopic array of topics—and by the fact that Americans have employed “privacy” to talk about all of them. If so, they would replicate my own research process. I went looking for privacy talk in the past and found it just about everywhere: in census schedules and public health campaigns, scientific laboratories and suburban design, marketing agencies and welfare bureaus, social movements and therapeutic sessions. By deliberately peering into otherwise unrelated domains in U.S. society, I aimed to piece together a new picture of how and why privacy came to matter so much to modern Americans.Although my book takes a capacious view of privacy’s history—ranging across topics from photography to policing, research ethics to “outing”—its emphasis may surprise some readers. The Known Citizen is neither a history of the surveillance society nor of the national security state, the two most common frameworks for thinking about privacy in the early twenty-first century. From where I sit, any student of this topic needs to reckon with the fact that citizens have always simultaneously resisted and craved being known, both pursued and dispensed with privacy. This makes the problem of privacy in American life—and the dilemma of the known citizen—both more complex and more mundane than headline-grabbing stories of data mining or government spying allow.One thing writing this book convinced me of is that privacy in modern America has not been “private” at all. It has instead functioned as a crucial category of public life. Precisely because privacy in the United States has been billed as a personal possession, outside the realm of the state or politics, its history opens up an illuminating window onto the social strains of modern citizenship.Privacy became a potent language in American life because it helped bridge the tension between expanding claims to personal inviolability and more sophisticated methods of infringing on it. It mediated the tug of war between the desire for anonymity and the longing for recognition, sometimes even within the same person. It was, in short, a tool for navigating an increasingly knowing society—one in which securing the boundary between one’s private affairs and one’s public identity was a routine, and yet urgent, task.For this reason, I part ways with those who argue that privacy is “dead” or “over”—a common refrain in the past decade (although it first appeared long before that!). Legal and political thinkers, but also ordinary Americans, have regularly remade privacy to meet new conditions. They have done so by rethinking the boundary between themselves and their society. My guess is that they will do so again, even in our own age of social media and big data.Who has the right to know? What ought to be publicly known? Who and what should remain unknown? These are the questions that American privacy debates sought to settle—at least provisionally—in the past. They are questions we will likely continue to wrestle with for the foreseeable future. I hope this book will inform the discussion.