Association studies involve accessing, parsing, generating, and analyzing large volumes of data, often carried out in many steps over many months. Large-scale surveys of genetic variation, such as the International HapMap Project, and rapidly increasing volumes of single-nucleotide polymorphism (SNP) genotyping data have created exciting opportunities for association studies. However, they have further exacerbated the difficulty of curating and analyzing such data. Haploview is a program developed in Mark Daly's lab at the Broad Institute of MIT and Harvard, which is designed to bundle many everyday analysis tasks into one easy-to-use package. Haploview has several features that are useful throughout different phases of association studies. Several of these features are illustrated in this article by following a hypothetical association study from design to execution. Haploview is used to (1) analyze HapMap data and choose tag-SNPs, (2) evaluate the quality of disease genotype data, (3) test for association, and (4) evaluate a region for follow-up of a positive association.