This article has provided a brief overview of the prevalence, differential diagnosis, and clinical manifestations of depression in the primary care setting. While the high prevalence of depression is well documented, another body of evidence is accruing that demonstrates that depression not only increases over utilization of medical resources, but may worsen the long-term prognosis of certain medical conditions such as MI. Evidence also suggests that the diagnostic and management skills of primary care physicians--who comprise the "front line" and may offer the only line of care for these patients--is in need of improvement. This series of articles focusing on the diagnosis and treatment of depression in primary care will hopefully contribute to that effort.