A patient-worn arrhythmia monitoring and treatment device weight between 250 grams and 2,500 grams includes at least one contoured pad configured to be adhesively coupled to a patient's torso, a plurality of therapy electrodes, at least one of which is integrated with the at least one contoured pad, and a plurality of ECG sensing electrodes, at least one of which is integrated with the at least one contoured pad. At least one housing configured to form a watertight seal with the at least one contoured pad extends no more than 5 cm from the contoured pad. A processor disposed within the housing is coupled to a therapy delivery circuit and configured to detect one or more treatable arrhythmias based on at least one ECG signal and cause a therapy delivery circuit to deliver at least one defibrillation pulse on detecting the one or more treatable arrhythmias.