Twenty mile per hour (20 mph) or 30 kph speed limit interventions are increasingly common in a wide range of European cities. Importantly, 20 mph or 30 kph speed limit interventions may not only reduce road danger, but also positively contribute to broader public health outcomes, such as active travel, play, and air quality. The aim of this chapter is to provide an updated review on the effects of 20 mph speed limits on a range of public health outcomes. To this end, electronic databases of academic literature were searched as well as the grey literature, including MEDLINE, EMBASE, Web of Science and Transport Research Information Service. Databases were searched using keywords related to ‘20 mph’ AND ‘health’. Thirteen academic studies and ten grey literature reports met the inclusion criteria. Clear impacts of 30 kph on reduced injuries, collisions and casualties were consistently reported, whereas less evidence is available on wider health impacts such as changes in active travel, play, and air quality. Few subgroup analyses were reported by age, gender, neighbourhood deprivation level, sex, ethnic minority, work status, disability, or type of user (e.g., cyclist, motorcyclist, car user, or pedestrian). Simultaneously, methodological challenges of existing studies were identified and discussed, including the challenge to assess wider health impacts of 20 mph speed limits and the difficulty in evaluating these limits using experimental methods, whilst also recognizing the system-wide context in which limits are introduced. Future studies should address these challenges and account for the broader role that lower speed limits can play. Without this, lower speed limits research will not unlock the full potential that lower speed limits may play in creating healthier, more inclusive, and more sustainable living environments.