The discipline of genetic epidemiology pertains to the vertical transmission of the susceptibility (predisposition) to a complex disease in a structured population. This statement meets halfway 1 the broad definitiongiven by N. E. Morton and S. c. Chung in 1978 2 and the concise one given by M. -C. King et al. in 1984. 1t pinpoints the fundamental genetic hypothesis, namely, the existence of an inherited condition that predisposes an individual to a specific disease, and the corresponding subject ofinvestigation, the family. Thus, the genetic epidemiological situation consists of three basic elements: (l) the genealogical structure, (2) the mode of inherit- ance (i. e. , the "genetic model") for the trait of interest, and (3) the observable phenotypes of susceptibility. It is clear that genetic epidemiology is a research field posi- tioned at the intersection of molecular genetics, population gen- etics, and clinical genetics. Perhaps the genealogical tree should be its central element: it evidences something forgotten in mole- cular genetics, namely the relationships, and associations with probabilistic and statistical concepts from population genetics. It offers a structure and a "history" for those clinicians studying familial diseases who are searching for genetic determinants of susceptibility. The genetic epidemiologist begins his analysis with a point on this genealogical tree, namely the proband, and attempts to carry out (nonrandom) "ascertainment sampling" by using a strategy that depends on the form and dimension (extended pedigrees versus nuclear families) of the tree.