This review examines the etiology and pathogenesis of idiopathic achalasia. This disease is clinically characterized by dysphagia of solids and liquids due to the presence of simultaneous or absent esophageal contractions and impaired or absent relaxation of the lower esophageal sphincter. It includes a review of (a) etiology and pathogenesis of this inflammatory process that damage the ganglion cells of the Auerbach plexus that is limited to the esophagus; (b) genetic abnormalities and polymorphisms associated with this disease that may help explain its heterogeneity expressed by the different motility abnormalities of its phenotypes as well as differences in its clinical progression. These different genetic abnormalities may be responsible for the slow progression of types I or II phenotypes; (c) indirect evidence of viruses present in these patients that may initiate its development; (d) the abnormalities of the muscle layer that may be responsible for the dilation of the body of the esophagus that ultimately causes the sigmoid-like esophagus in the very last phase of this disease. This progression to the end-stage phase tends to occur in about 5% of patients. And, (e) the chronic inflammatory abnormalities in the squamous mucosa that may be the cause of the dysplastic and neoplastic changes that may lead to squamous cell carcinoma whose incidence in this disease is increased. These mucosal abnormalities are usually present in patients with markedly dilated body of the esophagus and severe food stasis.