The first three years of life are critical for children’s long-term development, health and wellbeing, since core brain functions are formed during this period and the brain is much more vulnerable to adversity, which is key for the emergence of developmental disorders (autism, attention deficit/hyperactivity disorder). Identifying the mechanisms that act during early childhood to shape individual long-term social development is essential for improving the health and wellbeing and has farreaching consequences for social and educational policy. We propose that this key scientific problem cannot be solved without first understanding how early social exchanges tune critical brain functions in early childhood. Daily, repeated exchanges between infants and caregivers provide the substrate for the interacting influences of genes and the environment on brain development. By studying the micro-scale dynamics of early social exchanges we can revolutionise the understanding of macro-scale development of socially-relevant functions like theory of mind, language, attention, and emotion regulation. Our radically new approach puts social exchanges at the heart of brain development, which is sculpted through a complex ongoing cascade of interactions between the brain and its social environment. SAPIENS combines advances in ecological, mobile real-time brain and behaviour measurement, data modelling and artificial intelligence approaches to train a new generation of researchers in implementing these ground-breaking approaches to early social development. SAPIENS will generate revolutionary new tools for translating cutting-edge basic science into clinical diagnostic and treatment tools for developmental disorders, while engaging the public and delivering new public policy. By building on a strong intersectorial network of excellent research centres and industry, we will provide highly innovative and multidisciplinary training for young research leaders in developmental science.