Hypotonic lower esophageal sphincter (LES) and hiatal hernia are critical components in the pathogenesis of gastroesophageal reflux disease (GERD). These conditions involve anatomical and functional disruptions of the esophagogastric junction (EGJ), characterized by reduced LES pressure and displacement of the sphincter above the diaphragm. Patients present with a wide range of symptoms, from typical manifestations like heartburn and regurgitation to extra-esophageal symptoms such as cough and respiratory issues. Diagnosis requires a comprehensive approach, including detailed clinical assessment, physical examination, and multiple diagnostic tests. Key diagnostic techniques include upper gastrointestinal endoscopy, barium swallow study, high-resolution manometry (HRM), and reflux monitoring tests. Recent advances like the American Foregut Society classification and the Milan score have improved the precision of EGJ assessment. Endoscopy can identify hiatal hernia and definitive GERD criteria, while HRM provides crucial information about LES function and EGJ integrity. Reflux monitoring tests, particularly multichannel intraluminal impedance pH, quantify acid exposure and correlate symptoms with reflux events. The diagnostic algorithm emphasizes individualized assessment, with empiric proton pump inhibitor therapy for typical symptoms and immediate endoscopic evaluation for warning signs. Continued research is essential to enhance the understanding and management of these complex gastrointestinal disorders.