A rare disease, achalasia is the most frequent and best-known esophageal motility disorder. Its etiology is still unclear, but it is thought to result from a progressive loss of myenteric neurons at the cardia level. This leads to the impairment of relaxation of the lower esophageal sphincter (LES), a loss of peristalsis and a progressive dilation of the esophagus. Dysphagia is present in almost all patients; regurgitation, chest pain and weight loss are also frequent. Since dysphagia is an ominous symptom, endoscopy is the first test to be performed in these patients, revealing a somewhat dilated esophagus containing undigested food, as well as a closed cardia requiring gentle endoscope pressure to pass through it. Endoscopy may be negative, however, and further tests may be necessary for the diagnosis. Barium swallow usually reveals the characteristic “bird’s beak” appearance of the cardia, with a dilated esophagus: this allows a classification of achalasia into four stages, stage IV being the most advanced and difficult to treat. High-resolution manometry confirms the absence of LES relaxation and esophageal peristalsis. Moreover, it allows a clinically relevant classification of achalasia into three subtypes, of which subtype 3 is poorly responsive to pneumatic dilation and therefore requires surgery or peroral endoscopic myotomy.