The implantable cardioverter defibrillator, or "ICD", is a technologically challenging type of contemporary therapy. Engineers who design ICDs are being called upon by clinicians to extend the technological envelope in the quest of building the "ideal" device. Engineers require an appreciation of the real world "boundary conditions" and clinical impact of various ICD features, if designs are to be perfected. This text is intended to serve as an educational tool to foster mutual understanding and communication among physicians, engineers, and other professionals involved in ICD therapy, with the ultimate purpose of enhancing patient care.