Sick sinus syndrome has become a frequent and challenging diagnosis for the practicing physician (Fig. 6). Increased longevity will create even more patients in need of effective therapy. Multiple etiologies have been described, but the common final pathway in most instances appears to be chronic fibrosis of the sinus node. The multifaceted features of the syndrome have delayed an organized diagnostic approach until recently. Noninvasive diagnosis often suffices, particularly with the availability of Holter monitoring. Invasive studies help to confirm sinus node dysfunction and possible associated conduction defects in these patients. The prognosis is variable and at times unpredictable. Nonetheless, it is best to establish the diagnosis promptly and to institute appropriate therapy. Cardiac pacing has had a particularly beneficial impact on the long-term management of the sick sinus syndrome.