This paper gives a short overview of our recent research on changes in the body mass index (BMI) or in body fat for children studied longitudinally from birth to adulthood. The BMI shows characteristic changes in childhood and adolescence which are different from those known for skeletal growth. A period of loss of BMI from 1 to 6 years is followed by a pubertal spurt which is larger in females than in males. Fat shows a dramatic increase in the 1st year, and velocity is higher for girls than for boys after 3 years of age. At puberty there is a pre-pubertal and a post-pubertal spurt in total body fat or in arm fat and a dip in between. Those heavy or fat as adults have a qualitatively similar pattern of developmental changes, but much more accentuated fluctuations. The analysis relies on new statistical techniques.