Obesity is usually defined on the basis of deviations in cross-sectional population distribution of body mass. However, it is a heterogenous condition in terms of the changes of the individual body mass over time--there is a, probably life-long, tracking tendency, but also a considerable super-imposed age-dependent as well as age-independent variation over time. The tracking level as well as the fluctuation may be influenced by genes, environment, and gene-environment interactions. The longitudinal studies of subjects adopted-away early in life and their biological and adoptive relatives may be a particularly powerful method for assessment of the effects of genes and environment on obesity considered as a continuous trait both in terms of body mass and time.