Throughout the human genome, a correlation structure exists across genetic variation of different loci. Such a correlation structure means that knowing the genotype at one locus might provide information about the genotype at a second locus. This correlation between variation at different loci is termed linkage disequilibrium (LD). LD has implications in numerous avenues of genetic research. This article discusses the importance of LD in genetics, touching on both population genetics and association studies. It then introduces the seminal collaborative scientific endeavor to map LD in the human genome--the International HapMap Project--and its relevance for imputation.