Genome-wide association studies have opened a new era in the study of the genetic basis of common, multifactorial diseases and traits. Before the introduction of this approach only a handful of common genetic variants showed consistent association for any phenotype. Using genome-wide association, scores of novel and unsuspected loci have been discovered and later replicated for many complex traits. The principle is to genotype a dense set of common genetic variants across the genomes of individuals with phenotypic differences and examine whether genotype is associated with phenotype. Because the last common human ancestor was relatively recent and recombination events are concentrated in focal hotspots, most common variation in the human genome can be surveyed using a few hundred thousand variants acting as proxies for ungenotyped variants. Here, we describe the different steps of genome-wide association studies and use a recent study as example.