Unintentional weight loss is a problem encountered frequently in clinical practice. Weight loss and low body weight have potentially serious clinical implications. Although a nonspecific observation, weight loss is often of concern to both patients and physicians. There are multiple potential etiologies and special factors to consider in selected groups, such as older adults. A rational approach to these patients is based on an understanding of the relevant biologic, psychological, and social factors identified during a thorough history and physical examination. The goal of this article is to discuss the clinical importance, review potential pathophysiology, and discuss specific etiologies of unintentional weight loss that will enable the clinician to formulate a practical stepwise approach to patient evaluation and management.