When enough females and males are enrolled in clinical trials, much more relevant information is available on potential sex differences in device safety and effectiveness. Unfortunately, females have largely been underrepresented in clinical studies of cardiac medical device therapies for heart failure. In this chapter, sex differences in heart failure characteristics and cardiac electrophysiology and their influence on the safety, effectiveness, and application of implantable cardioverter defibrillators (ICDs), subcutaneous ICDs (S-ICDs), and cardiac resynchronization therapy (CRT) will be discussed. In this way, the research community will hopefully become more appreciative of the potential differences in device effects between females and males.