Aim: To summarize the evidence on clinical effectiveness and safety of wearable cardioverter defibrillator (WCD) therapy for primary and secondary prevention of sudden cardiac arrest in patients at risk., Methods: We performed a systematic literature search in databases including MEDLINE via OVID, Embase, the Cochrane Library, and CRD (DARE, NHS-EED, HTA). The evidence obtained was summarized according to GRADE methodology. A health technology assessment (HTA) was conducted using the HTA Core Model ® for rapid relative effectiveness assessment. Primary outcomes for the clinical effectiveness domain were all-cause and disease-specific mortality. Outcomes for the safety domain were adverse events (AEs) and serious adverse events (SAEs). A focus group with cardiac disease patients was conducted to evaluate ethical, organizational, patient, social, and legal aspects of the WCD use., Results: No randomized- or non-randomized controlled trials were identified. Non-comparative studies (n=5) reported AEs including skin rash/itching (6%), false alarms (14%), and palpitations/light-headedness/fainting (9%) and discontinuation due to comfort/lifestyle issues (16-22%), and SAEs including inappropriate shocks (0-2%), unsuccessful shocks (0-0.7%), and death (0-0.3%). The focus group results reported that experiencing a sense of security is crucial to patients and that the WCD is not considered an option for weeks or even months due to expected restrictions in living a "normal" life., Conclusion: The WCD appears to be relatively safe for short-to-medium term, but the quality of existing evidence is very low. AEs and SAEs need to be more appropriately reported in order to further evaluate the safety of the device. High-quality comparative evidence and well-described disease groups are required to assess the effectiveness of the WCD and to determine which patient groups may benefit most from the intervention., Competing Interests: Disclosure The authors report no conflicts of interest in this work.