In brief:The author describes symptoms, diagnostic tests, and follow-up treatment and advice for 12 cases to illustrate cardiac rhythm problems in runners. After deciding if the disturbance is abnormal or merely a variant of normal that can occur in an athletic person, the physician should search for associated underlying cardiac disorders such as hypertrophic obstructive cardiomyopathy, mitral valve prolapse, or coronary disease that might predispose the person to an arrhythmia. Noninvasive methods, including a thorough cardiovascular physical examination, exercise testing, stress scans, ambulatory monitoring, cardiac fluoroscopy, and echocardiography, should be used whenever possible.