PURPOSE: the aim of this study was to systematically review the literature on accelerometer-measured physical activity and health outcomes in adults. METHODS: Eight electronic databases were searched for relevant articles published up to March 2021. Only population-based studies of adults (age ≥18 yr) that directly compared two or more categories of physical activity (i.e. bout duration, intensity, and daily/weekly frequency) with a health outcome (e.g. mortality, cardiometabolic, healthy aging, depression, sleep, and brain structure) were included. RESULTS: of the 15,923 publications retrieved, 52 articles were included. Twenty-eight studies directly compared the associations between physical activity accumulated in different bout durations, 31 studies directly compared the associations between physical activity accumulated in different intensities, and 9 studies directly compared the associations between the effects of varying daily and weekly frequencies of physical activity, with health outcomes. Most showed no differences in relationships with health outcomes when physical activity was accumulated in short (