Quantitative variation underlies normal as well as pathological traits, and large part of this variability is under the control of genetic loci. Thanks to a better understanding of the extent and nature of human genetic variability and the subsequent availability of an increasing number of genetic markers, genetic mapping of several such quantitative trait loci, or QTLs, has been accomplished in the past 20 years or so using linkage and association analysis in family-based and population-based studies. Rather than alternative, such methods are complementary as each has optimal power of detecting genetic variants underlying variability of quantitative traits under different scenarios defined by the QTL allele frequencies and magnitude of genetic effects. We describe how to apply such analyses to whole-genome or candidate-gene genetic marker data to correlate genetic variability to quantitative trait variability for the purpose of gene mapping and identification.