Complex genetic traits are some of the most challenging in human studies. Such traits, which are characterized by genetic and etiologic heterogeneity, typically show evidence of familial influences, but do not follow simple Mendelian patterns of inheritance. Of interest in this chapter are complex traits that are measured on a scale that permits analysis as quantitative traits. A few examples are Alzheimer’s disease (where age at onset is the continuous trait), various lipid levels, learning disabilities, and hypertension. The goal is to map quantitative trait loci (QTLs) that contribute to such traits.