During the past 3 decades, more research has been conducted on depression than any other psychiatric disorder. While there are numerous studies on depression in the areas of epidemiology, biopsychosocial correlates, genetics, course, and treatment, remarkably little research has focused on the criteria used to diagnose major depressive disorder. Nearly 10 years ago, we began the Rhode Island Methods to Improve Diagnostic Assessment and Services (MIDAS) project, an integration of a comprehensive assessment protocol into a community-based psychiatry outpatient practice. As part of this clinical-research program, patients have been administered semistructured diagnostic interviews to assess the DSM-IV Axis I and Axis II disorders by well-trained interviewers. One of the goals of the MIDAS project has been to examine issues of nosology. While changes in the successive editions of the American Psychiatric Association's DSM have been increasingly grounded on empirical research, many of the sets of diagnostic criteria have not been subject to empirical scrutiny. During the next 12 months, we will be publishing a series of papers examining varying aspects of the diagnostic criteria for major depression. This series represents the type of methodical psychometric and conceptual analysis that should be conducted when developing or revising diagnostic criteria. We will examine whether the assumptions underlying the DSM's diagnostic rules have empirical support, and examine the impact of these rules on clinical practice. Our goal is not to develop a new definition of depression that would classify some individuals differently than how they are currently classified. That is, we are not seeking to develop a more valid definition of depression. However, our findings sometimes suggest changes that would simplify the criteria, improve their psychometric properties, and enhance their applicability in medical settings. Thus, the focus in the series is on clinical utility rather than validity.