Genetic architecture refers to the numbers and genome locations of genes that affect a trait, the magnitude of their effects, and the relative contributions of additive, dominant, and epistatic gene effects. Quantitative trait locus (QTL) mapping techniques are commonly used to investigate genetic architectures, but the scope of inferences drawn from QTL studies are often restricted by the limitations of the experimental designs. Recent advances in experimental and statistical procedures, including the simultaneous analysis of QTL that segregate in diverse germplasm, should improve genetic architecture studies. High-resolution QTL mapping methods are being developed that may define the specific DNA sequence variants underlying QTL. Studies of genetic architecture, combined with improved knowledge of the structure of plant populations, will impact our understanding of plant evolution and the design of crop improvement strategies.