Complex polygenic and multifactorial diseases remain a challenge for human geneticists. Here we aim to remind basic definitions of multifactorial diseases and the genetic related concepts underlying classical methods. Knowledge on pathophysiological process and the genetic information available conditions the design of study. The choice of methodology, between candidate gene approach and genome scan approach, between linkage and association studies, is the most important step. Both methods, linkage analysis and association studies are usually considered as complementary approaches for a given disease. For this reason, in this article, we present the most important classical methodologies in genetic epidemiology of complex disorders. References and examples are given to illustrate.