A strong resemblance exists between the height of family members, and the extent of this similarity increases as the family relationship becomes closer. This suggests that there is a genetic basis to adult height. The genetic basis of height is particularly apparent in monozygotic twins who have been reared apart. Although it is difficult to separate completely the genetic and environmental components that influence growth, there does appear to be a strong genetic influence on final height achieved and the timing of key stages in the growth pathway.