Definitions of risk, types of risk factors used in longitudinal studies of the development of infants and toddlers (0-3 years), and the predictive power of risk models in assessing developmental delays are reviewed Biologic factors in combination with environmental risk factors give the best prediction of long-term outcome When biological or environmental risk factors are examined independently, they are not powerful predictors Recommendations are made for repeated screening for developmental risk factors during the first 3 years, and a model for provision of services is presented that assumes a differential risk algorithm.