In chapter one, I tell the story of how the recent controversy regarding the meaning of mental illness erupted.Chapter two addresses whether we are pathologizing everyday life. I focus on the boundary problem—the difficulty distinguishing distress from disorder within our current system. I consider how economic and political forces tug at this boundary, affecting how we understand sexual dysfunction, depression, social anxiety disorder, and posttraumatic stress disorder (PTSD) reportedly arising from watching televised images of the 9/11 terrorist attacks, for example.Asking whether evolutionary psychology can make sense of mental disorder, in chapter three, I analyze an influential approach to conceptualizing mental disorder as harmful dysfunction in evolved psychobiological mechanisms of the mind. Noting the limits of this approach, I suggest a promising revised version.In chapter four, I scrutinize attempts to solve an apparent evolutionary paradox: If mental disorders are heritable, common, and harmful, why hasn’t natural selection eliminated genetic variants predisposing to disorder from the population? I show how two theorists have provided a convincing solution to this puzzle.Chapter five provides an analysis of social constructionist approaches to mental disorder. Social constructionist theorists question whether mental health professionals discover mental disorders in the same way as nonpsychiatric physicians discover infectious diseases. They suggest instead that social processes, not biological ones, shape or create mental disorders. Drawing on historical and cross-cultural scholarship, I show how these theories vary in plausibility, depending on the disorder.In chapter six, I cover the fast-moving field of psychiatric genomics, illustrating how it is changing our understanding of the role of genes in risk for mental illness.In chapter seven, I draw on recent work in psychology and philosophy of science designed to answer whether mental disorders differ by kind or degree.Closing the book, chapter eight provides some provisional answers to the driving question—What is mental illness?What is mental illness?There is no sound-bite answer to this question. We will never have a clear-cut set of criteria that identifies all instances of mental disorder and excludes everything else.The reason for this is that we do not discover mental illnesses in nature as scientists have done with the elements of the periodic table.Nevertheless, there are facts about how the mind/brain produces psychiatric symptoms, and these are discoverable by science.Ideally, science can illuminate what has gone with a person and can inform us about what treatments will work best to fix it. These facts provide the basis for sound arguments about where to draw the boundary between distress and disorder even though social, political, cultural, and economic factors will always play a role in this process.