Technical Abstract: Genome-wide association studies (GWAS) may benefit from using haplotype information for making marker-phenotype associations. Several rationales for grouping single nucleotide polymorphisms (SNPs) into haplotype blocks exist, but any advantage may depend on the genetic architecture of traits, patterns of linkage disequilibrium in the population, and marker density. The objective of this study was to explore the utility of haplotypes for GWAS in barley (Hordeum vulgare) to offer a first detailed look at this issue in crops. We used genotype and phenotype data from the Barley Coordinated Agricultural Project and constructed haplotypes using three different methods. When QTL were simulated using single SNPs dropped from the marker dataset, a single SNP analysis performed at least as well in terms of power and false discovery rate as any of the haplotype analyses. However, the haplotype analyses performed better 1) when QTL were simulated as polymorphisms that arose subsequent to marker variants, and 2) in analysis of empirical heading date data. These results demonstrate that the information content of haplotypes is dependent on the particular mutational and recombinational history of the QTL and nearby markers. Analysis of the empirical data also confirmed our intuition that the distribution of QTL alleles in nature is often unlike the distribution of marker variants, and hence utilizing haplotype information could capture associations that would elude single SNPs. We recommend routine use of both single SNP and haplotype markers for GWAS to take advantage of the full information content of the genotype data.