In this chapter, we provide a brief introduction to our book. We discuss the following themes, which run throughout this edited book on depressive disorders and comorbidity: assessment and diagnosis, theory and methods, psychiatric comorbidity, health comorbidity, relationship comorbidity, intervention and consultation, and future directions. A number of themes will be apparent, including the incredibly broad scope of depressive comorbidity. Depression goes with many other problems. Another theme is that the specifics of depressive comorbidity—and the implications for theory, research, and practice—vary considerably as we consider one type of comorbidity versus another. For example, the comorbidity of depression and generalized anxiety disorder has very different implications than the comorbidity of depression and alcohol-use disorder, which in turn is different than the comorbidity of depression and cancer, which again has different implications than the comorbidity of depression and severe relationship dysfunction. Each of the chapters in the book highlight some of the themes and issues, but the remarkable breadth and depth of depressive comorbidity becomes clearer as we consider all of the chapters in total. We attempt to bridge some of these differences and look for common themes in the Epilogue at the end of the book, as do some of the contributors in their individual chapters on specific issues or types of comorbidity. In this Introduction, however, we focus more on the specific chapters and a few of the themes that are highlighted in each one. Overarching themes, such as what is meant bycomorbidity, how might future efforts at assessment and treatment be improved, and what future developments may be particularly helpful are discussed in many of the individual chapters. This brief introduction serves to highlight a few of the issues and introduce the reader to the broad array of chapters that await them in the rest of the book.