Growth velocity is higher in late intra-uterine and early post-natal life than at any time thereafter, and accurate measurements are essential for appropriate monitoring. The accuracy with which such measurements are made and recorded is frequently questionable, however, and short- and medium-term changes in growth may be difficult to interpret in the light of normal variations in the pattern of growth. Infants who are small at birth must be accurately classified because intra-uterine growth retardation and small for gestational age have different implications for both causation and outcome. Prediction of expected growth on the basis of mid-parental height is essential but frequently omitted. Post-natal growth impairment is common in pre-term infants and is often rapid in onset. Poor growth may continue for many months, and catch-up may be incomplete. Early growth failure may have a significant influence on subsequent morbidity and mortality.