This chapter overviews markers of infant health during the twentieth century and presents a descriptive account of current social and ethnic differences in markers of infant health. Infants born with weights at the lower end of the birthweight distribution are at increased risk of adverse outcomes, and there is a distinctive reverse-J-shaped relationship between birthweight and mortality during the first year of life. In order to separate the effects of birthweight and length of gestation out from the notion of 'prematurity' it was suggested in the early 1970s that gestational age be categorised as follows: pre-term, term and post-term. A wealth of scientific evidence has highlighted the benefits of breastfeeding for infants and mothers. As ethnic minority groups in the UK appear to have a breastfeeding advantage compared with the ethnic majority, future policies aimed at increasing breastfeeding rates need to pay attention to the different social, economic and cultural profiles of all ethnic groups.