A wearable external defibrillator comprising: a first patient engagement substrate comprising an adhesive, one or more sensing electrodes, and a first defibrillator electrode pad, the one or more sensing electrodes configured to engage with a patient's skin to detect a cardiac signal, the first defibrillator electrode pad configured to engage with the patient's skin and to deliver an electrical therapy to the patient, the first defibrillator electrode pad configured to be in continuous electrical communication with the patient's skin; one or more waterproof housings connected to the first patient engagement substrate to form an interior space between the one or more housings and the first patient engagement substrate a battery, one or more capacitors, and a controller disposed in the interior space, wherein the first defibrillator electrode pad is in electrical communication with the one or more capacitors; a second patient engagement substrate comprising a second defibrillator electrode pad, and a second adhesive, the second defibrillator electrode pad configured to engage with the patient's skin and to deliver an electrical therapy to the patient, the second defibrillator electrode pad configured to be in continuous electrical communication with the patient's skin, the second defibrillator electrode in electrical communication with the one or more capacitors; and a fluid transport element configured to transport fluid away from the skin to allow the wearable external defibrillator to be worn continuously for greater than 24 hours, wherein the controller is configured to charge the one or more capacitors with the battery and to discharge the one or more capacitors through the first defibrillator electrode pad and the second defibrillator electrode pad, characterised in that said second patient engagement substrate further comprises a plurality of sensing electrodes. a first patient engagement substrate comprising an adhesive, one or more sensing electrodes, and a first defibrillator electrode pad, the one or more sensing electrodes configured to engage with a patient's skin to detect a cardiac signal, the first defibrillator electrode pad configured to engage with the patient's skin and to deliver an electrical therapy to the patient, the first defibrillator electrode pad configured to be in continuous electrical communication with the patient's skin; one or more waterproof housings connected to the first patient engagement substrate to form an interior space between the one or more housings and the first patient engagement substrate a battery, one or more capacitors, and a controller disposed in the interior space, wherein the first defibrillator electrode pad is in electrical communication with the one or more capacitors; a second patient engagement substrate comprising a second defibrillator electrode pad, and a second adhesive, the second defibrillator electrode pad configured to engage with the patient's skin and to deliver an electrical therapy to the patient, the second defibrillator electrode pad configured to be in continuous electrical communication with the patient's skin, the second defibrillator electrode in electrical communication with the one or more capacitors; and a fluid transport element configured to transport fluid away from the skin to allow the wearable external defibrillator to be worn continuously for greater than 24 hours, wherein the controller is configured to charge the one or more capacitors with the battery and to discharge the one or more capacitors through the first defibrillator electrode pad and the second defibrillator electrode pad, characterised in that said second patient engagement substrate further comprises a plurality of sensing electrodes.