Apple Heart Study: Assessment of Wristwatch-Based Photoplethysmography to Identify Cardiac Arrhythmias

Inclusion Criteria: Possession of the following at time of eligibility screening, ascertained from automatic hardware/software/device pairing check: * iPhone (5s or later) with iOS version 11.0 or later defined as iPhone model/iOS version used to complete screening eligibility. * Apple Watch (Series 1 or later) with watchOS version 4.0 or later defined as Apple Watch model/watchOS paired with iPhone used to complete screening eligibility. * Current resident of the United States at time of eligibility screening, defined by self-reported state of residence within the 50 states of the United States or District of Columbia. * Proficient in written and spoken English, defined by self-report of comfort reading, writing, and speaking English on iPhone. * Valid phone number associated with iPhone, ascertained from self-report. * Valid email address, ascertained from self-report. Exclusion Criteria: * Self-reported diagnosis of Atrial Fibrillation. * Self-reported diagnosis of Atrial Flutter. * Currently on anti-coagulation therapy.