Advances in the field of defibrillation have brought to practice different types of devices that include the transvenous implantable cardioverter-defibrillator (ICD) with or without cardiac resynchronization therapy, the subcutaneous ICD (S-ICD), and the wearable cardioverter-defibrillator. To ensure optimal use of these devices and to achieve best patient outcomes, clinicians need to understand how these devices work, learn the characteristics of patients who qualify them for one type of device versus another, and recognize the remaining gaps in knowledge surrounding these devices. The transvenous ICD has been shown in several randomized clinical trials to improve the survival of patients resuscitated from near-fatal ventricular fibrillation and those with sustained ventricular tachycardia with syncope or systolic heart failure as a result of ischemic or nonischemic cardiomyopathy despite receiving guideline-directed medical therapy. Important gaps in knowledge regarding the transvenous ICD involve the role of the ICD in patient subgroups not included, or not well represented, in clinical trials and the need to refine the selection criteria for the ICD in patients who are indicated for it. S-ICDs were recently introduced into the clinical arena as another option for many patients who have an approved indication for a transvenous ICD. The main advantage of the S-ICD is a lower risk of infection and lead-related complications; however, the S-ICD does not offer bradycardia or antitachycardia pacing. The S-ICD may be ideal for patients with limited vascular access, high infection risk, or some congenital heart diseases. However, more data are needed regarding the efficacy and effectiveness of the S-ICD in comparison to transvenous ICDs, the extent of defibrillation testing required, and the use of the S-ICD with other novel technologies, including leadless pacemakers. Cardiac resynchronization therapy-defibrillators are indicated in patients with a left ventricular ejection fraction ≤35%, QRS width ≥130 ms, and New York Heart Association class II, III, or ambulatory IV symptoms despite treatment with guideline-directed medical therapy. Multiple randomized controlled trials have shown that the cardiac resynchronization therapy-defibrillator improves survival, quality of life, and several echocardiographic measures. One main challenge related to cardiac resynchronization therapy-defibrillators is the 30% nonresponse rate. Many initiatives are underway to address this challenge including improved cardiac resynchronization therapy and imaging technologies and enhanced selection of patients and device programming.