The World Health Organization Multicenter Growth Reference Study is a multinational project to develop new growth references for infants and young children. The design combines a longitudinal study from birth to 24 months with a cross-sectional study of children aged 18 to 71 months. The pooled sample from the six participating countries (Brazil, Ghana, India, Norway, Oman, and the United States) consists of 8440 children. The new WHO Child Growth Standards confirm that all children worldwide, given an optimum start in life, have the same potential for growth and prove that differences in children's growth to the age of 5 years are more influenced by nutrition, feeding practices, environment, and healthcare than by genetics or ethnicity. The new standards are based on the breast fed child as the norm for growth and development. For the first time, this ensures coherence among the tools used to assess growth and national and international infant feeding guidelines, which recommend breast feeding as the optimal source of nutrition during infancy.