In this paper the Generation R Study is presented. This study examines growth, development and health in urban children from fetal life until young adulthood. With an integrated approach of epidemiological, clinical and basic research, it focuses on four primary areas of research: (1) growth and physical development; (2) behavioural and cognitive development; (3) diseases in childhood; and (4) health and health care for pregnant women and children. The general aims of the study are: 1. to describe normal and abnormal growth, development and health from fetal life until young adulthood in a multiethnic population-based cohort; 2. to identify biological, social and environmental determinants of normal and abnormal growth, development and health from fetal life until adulthood; 3. to examine the utilisation and effectiveness of current strategies for prevention and early identification of groups at risk. Eventually, this study will contribute to the development of strategies for optimising health and health care for pregnant women and children. The Generation R Study is a prospective population-based cohort study in Rotterdam, the Netherlands. In this urban setting, 10 000 children will be examined from early fetal life until young adulthood. Data are collected by physical examinations, questionnaires, interviews, ultrasound and biological samples. The study entered its pilot phase to test the logistics in December 2001. Full participant recruitment and complete data collection started in 2002.