Infant feeding practices exert recognized effects on infant survival and command close attention from pediatricians, nutritionists, public health workers, demographers, social scientists, and parents. Despite considerable research and public health efforts, models to inform decision-making about timing of supplementation in policy and practice remain unsatisfactory. In the present review, new models are derived through analysis of the weanling's dilemma. After approximately 6 months, breast milk alone cannot meet the infant's nutritional requirements for growth, but nutritional supplementation raises risks of illness and malnutrition. Timing of supplementation is conditioned by complex short- and long-term trade-offs among infant and maternal needs and constraints. No formulaic solution can be prescribed for this dilemma, but optimal timing of supplementation can be ascertained from infant-, maternal-, and locale-specific conditions. Models for these determinants and trade-offs are presented and applied to data from longitudinal studies of breastfeeding, infant development, and interbirth interval.