At the time of writing, there is widespread concern about rising rates of mental distress and ill-health, particularly in some groups. This concern increased rapidly during the Covid-19 pandemic, with some commentators predicting huge increases in mental ill-health. Subsequent economic crises in many countries, rising inequalities, the looming climate catastrophe, and growing political instability, polarisation, and conflict, both within and between countries, further compound these concerns. Questions abound about the nature and extent of a mental health crisis across populations, social groups, and time; about the causes of increasing rates, e.g. about the complex interplay of societal, interpersonal, individual, and biological processes; and about what should or can be done to address the challenge of rising rates, at policy, community, and service levels. These types of questions – of the distribution and causes of mental distress and ill-health in populations and of how to respond and intervene – are precisely the questions epidemiology, the science of population health, is concerned with. These questions are fundamental to our understandings of and responses to mental distress and ill-health.