Major environmental hazards such as ambient air pollution, environmental tobacco smoke, water and food contaminants, noise, pesticides and ultraviolet (UV) light may lead to serious, chronic pathologies with large societal and economic costs, especially when exposure occurs during critical periods of development in pregnancy or early life. The Exposome concept was coined to encompass the totality of human environmental exposures from conception onwards, complementing the genome. The key focus of the exposome is on its epidemiological applications for prevention of human disease. The HELIX project aims to exploit novel tools and methods to characterise early-life exposure to a wide range of environmental hazards, and integrate and link these with data on major child health outcomes, thus developing an Early-Life Exposome approach. HELIX uses six existing, prospective birth cohort studies as the only realistic and feasible way to obtain the comprehensive, longitudinal, human data needed to build this early-life exposome. These cohorts have already collected large amounts of data as part of national and EU-funded projects. Results will be integrated with data from European cohorts (>300,000 subjects) and registers, to estimate health impacts at the large European scale.