Body fatness is the result of a gradual process covering the whole period of growth. In this connection, one point of particular importance, which is in agreement with the view originally held by Brook, is that there is no clear-cut border between so-called normal body fatness and body leanness or obesity. In fact, body fatness offers a continuous spectrum and this continuum involves both fat cell size and number; the result varies with each individual depending on how the factors acting on adipose tissue development combined during the successive steps in the growth period.