Most traits of medical relevance follow quantitative inheritance patterns. The genetic dissection of quantitative traits poses special challenges for geneticists mainly because of low penetrance and gene-gene and gene-environment interactions. Emerging genome resources and technologies are enabling systematic investigation of the genetic architecture of quantitative traits in more efficient ways. This article summarizes the current state of medical quantitative trait locus (QTL) mapping-describing the methods, limitations, and achievements in the detection and characterization of QTLs.