This document discusses the use of echocardiography in selecting and evaluating patients for cardiac resynchronization therapy (CRT). It defines CRT as a biventricular pacemaker that sends electrical signals to both lower chambers of the heart. Guidelines recommend CRT for patients with heart failure, reduced ejection fraction, and prolonged QRS duration. Echocardiography can assess dyssynchrony through parameters like septal-to-posterior wall motion delay, left ventricular electromechanical delay, and apical rocking. CRT success is defined as ≥10-15% reduction in end-systolic volume after 3-6 months. The document reviews echocardiographic techniques for evaluating dyssynchrony and