Mental illness accounts globally for one third of years lived with disability, leading to tremendous loss of human, societal and economic potential. Most symptoms (e.g., depression, anxiety, conduct problems) emerge before adulthood, highlighting the first two decades of life as an important period of heightened vulnerability. Epigenetics has emerged as a biological system that captures the underlying genetic and environmental risk factors, but we do not yet understand the developmental dynamics in this system over time. Such knowledge, however, is critical if we are to understand how mental health disorders develop, and thus how they may be prevented.