Esophageal achalasia, characterized by failure of the lower esophageal sphincter to relax normally with swallowing and esophageal aperistalsis, may be primary or secondary to another disorder (in the United States most often cancer). Primary achalasia is of unclear etiology but almost certainly is a disorder of the innervation of the smooth muscle portion of the esophagus. This article reviews the classification and clinical features of achalasia syndromes, as well as current concepts of pathogenesis, diagnosis, complications, and therapy of this group of disorders.