The challenges of early diagnosis are similar in psychiatry to the rest of medicine. For potentially severe and persistent disorders there is great value in early diagnosis, however, only under certain conditions. Early diagnosis would not be justified if there were no efficacious treatments or if such treatments provided earlier would do more harm than good for those exposed. There is growing evidence that earlier and sustained intervention improves at least short-term outcomes. However, early intervention may be iatrogenic where systems of care are poor in quality. One thing is clear, the general pattern of care is still ‘too little, too late’ even in the most affluent countries. Consistent and extensive reform of health systems, with recognition of early intervention as an increasingly evidence-based ‘best buy’, represents one of the key priorities in international mental health.