EpiTIME aims to shine a light on the newly discovered epigenetic ‘time puzzle’ of child mental health. Recently, it has been observed that common mental health problems in children, such as inattention-hyperactivity and impulse-control problems, are most strongly predicted by epigenetic patterns regulating gene expression at birth – a signal that is curiously lost when measuring these same patterns later in childhood. Such a finding points to the existence of an early biologically-sensitive developmental window and may provide us with crucial insights into the nature and origins of mental health outcomes in children. Yet, how these epigenetic timing effects arise, what factors drive them and why they manifest is currently a puzzle. To solve it, this project will combine (i) the application of innovative, multidisciplinary approaches and (ii) the generation of new data within a unique set of European longitudinal cohorts to systematically characterize, locate and explain epigenetic timing effects on child mental health with unprecedented scale and depth. As well as addressing a major knowledge gap and advancing research at the forefront of biological and psychological sciences, EpiTIME has the potential to set in motion a paradigm shift in the way that we conceptualize, understand and approach mental health in children.