The basis for recent developments on the characterization of the linkage-disequilibrium
structure of the genome and the application of association mapping to genes for common
human diseases is described. Patterns of linkage disequilibrium are now understood, for a
number of human populations, in unprecedented detail. This information not only provides
a vital resource for the design and execution of powerful association-mapping studies, but
opens new avenues of research into the genetic history of human populations and the
effects of natural selection, mutation, and recombination on the genomic landscape.