This review focuses on methodological aspects and main results of different family studies that have been conducted to assess the existence of a genetic contribution in human obesity. A genetic component in the etiology of obesity has been elucidated through specific study designs answering different research questions such as: a) Do obesity aggregate in families? b) Is there a genetic contribution to familial clustering? c) Is it possible to localize chromosomal regions that contain susceptibility genes to obesity? d) Is it possible to estimate the risk for developing obesity depending on the genotype profile in candidate genes? There are sufficient evidences indicating the existence of a moderate familial clustering of obesity defined as body mass index >/=30 with a stronger aggregation with more extreme values of body mass index. Twin studies have demonstrated that the familial aggregation of obesity has a genetic component and is not only due to cultural or environmental factors clustered in families. Linkage studies have identified markers and genes related to obesity in virtually all human chromosomes. However, some of these linkage studies have produced conflicting results. Discordant results are even more pronounced in case-control studies that evaluate the association between alleles at candidate genes and obesity. Topics related to study design will acquire increasing importance in order to avoid methodological problems related to trait definition, sample sizes, population stratification by ethnicity and other confounding factors.