This current opinion provides an overview of the emerging discipline of muscle-strengthening exercise epidemiology. First, we define muscle-strengthening exercise, and discuss its recent addition into the global physical activity guidelines, which were historically mainly focused on aerobic physical activity (walking, running, cycling etc.). Second, we provide an overview of the current clinical and epidemiological evidence on the associations between muscle-strengthening exercise and health, showing a reduced mortality risk, and beneficial cardiometabolic, musculoskeletal, functional and mental health-related outcomes. Third, we describe the latest epidemiological research on the assessment, prevalence, trends and correlates of muscle-strengthening exercise. An overview of recent population estimates suggests that the proportion of adults meeting the current muscle-strengthening exercise guideline (10-30%; ≥ 2 sessions/week) is far lower than adults reporting meeting the aerobic exercise guideline (~ 50%; ≥ 150 min/week). Fourth, we discuss the complexity of muscle-strengthening exercise promotion, highlighting the need for concurrent, coordinated, and multiple-level strategies to increase population-level uptake/adherence of this exercise modality. Last, we explore key research gaps and strategies that will advance the field of muscle-strengthening exercise epidemiology. Our objective is to provide a case for increased emphasis on the role of muscle-strengthening exercise for chronic disease prevention, and most importantly, stimulate more research in this currently understudied area of physical activity epidemiology.