A wearable external defibrillator comprising: one or more sensing electrodes configured to engage with a patient's skin to detect a cardiac signal; a first defibrillator electrode pad (140, 202) 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 contact with the patient's skin; a first patient engagement substrate comprising an adhesive (128, 208), the first defibrillator electrode pad (140, 202), a first fluid transport element (142, 200) configured to transport fluid away from the skin to allow the wearable external defibrillator to be worn continuously, and a first vapor permeable layer; a second patient engagement substrate comprising a second defibrillator electrode pad, a second adhesive, a second fluid transport element (142, 200) in fluid communication with the second patient engagement substrate configured to transport fluid away from the skin to allow the wearable external defibrillator to be worn continuously, and a second vapor permeable layer; an energy source; one or more capacitors (156, 158) in electrical communication with the energy source and the first defibrillator electrode pad (140, 202) and the second defibrillator electrode pad; and a controller configured to detect the cardiac signal with the one or more sensing electrodes and to charge the one or more capacitors (156, 158) with the energy source followed by discharging the one or more capacitors (156, 158) to deliver a therapeutic shock through the first defibrillator electrode pad (140, 202) and the second defibrillator electrode pad to the patient while the first and second patient engagement substrates (102, 104) are engaged with the patient; and a housing, characterized in that the housing comprises a plurality of compartments (106a-d) supported by at least one of the first patient engagement substrate and the second patient engagement substrate, the compartments (106a-d) configured to reversibly receive and support the energy source, the one or more capacitors (156, 158), and the controller; and the wearable external defibrillator further comprises one or more electrical connectors (107a-d, 224, 226) configured to reversibly engage with the energy source, the one or more capacitors (156, 158), and the controller, the one or more electrical connectors (107a-d, 224, 226) supported by the first patient engagement substrate, the second patient engagement substrate and/or the compartments (106a-d). one or more sensing electrodes configured to engage with a patient's skin to detect a cardiac signal; a first defibrillator electrode pad (140, 202) 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 contact with the patient's skin; a first patient engagement substrate comprising an adhesive (128, 208), the first defibrillator electrode pad (140, 202), a first fluid transport element (142, 200) configured to transport fluid away from the skin to allow the wearable external defibrillator to be worn continuously, and a first vapor permeable layer; a second patient engagement substrate comprising a second defibrillator electrode pad, a second adhesive, a second fluid transport element (142, 200) in fluid communication with the second patient engagement substrate configured to transport fluid away from the skin to allow the wearable external defibrillator to be worn continuously, and a second vapor permeable layer; an energy source; one or more capacitors (156, 158) in electrical communication with the energy source and the first defibrillator electrode pad (140, 202) and the second defibrillator electrode pad; and a controller configured to detect the cardiac signal with the one or more sensing electrodes and to charge the one or more capacitors (156, 158) with the energy source followed by discharging the one or more capacitors (156, 158) to deliver a therapeutic shock through the first defibrillator electrode pad (140, 202) and the second defibrillator electrode pad to the patient while the first and second patient engagement substrates (102, 104) are engaged with the patient; and a housing, characterized in that the housing comprises a plurality of compartments (106a-d) supported by at least one of the first patient engagement substrate and the second patient engagement substrate, the compartments (106a-d) configured to reversibly receive and support the energy source, the one or more capacitors (156, 158), and the controller; and the wearable external defibrillator further comprises one or more electrical connectors (107a-d, 224, 226) configured to reversibly engage with the energy source, the one or more capacitors (156, 158), and the controller, the one or more electrical connectors (107a-d, 224, 226) supported by the first patient engagement substrate, the second patient engagement substrate and/or the compartments (106a-d).