Variation in early-life conditions can trigger developmental switches that lead to predictable individual differences in adult behaviour and physiology. Despite evidence for such early-life effects being widespread both in humans and throughout the animal kingdom, the evolutionary causes and consequences of this developmental plasticity remain unclear. The current issue aims to bring together studies of early-life effects from the fields of both evolutionary ecology and biomedicine to synthesise and advance current knowledge of how information is used during development, the mechanisms involved, and how early-life effects evolved. We hope this will stimulate further research into early-life effects, improving our understanding of why individuals differ and how this might influence their susceptibility to disease. This article is part of the theme issue ‘Developing differences: early-life effects and evolutionary medicine’.