Quantitative genetics in conjunction with statistics has provided much of the scientific framework for modern plant breeding. Although there has been no specific review of the contributions of quantitative genetics and statistics to plant breeding, the contributions have been undoubtedly profound and lasting. Quantitative genetic theory in many ways is robust to and naive of modern genetic principles. Little is known about the biology or the genetic architecture of quantitative traits. In this paper, five major areas of quantitative genetics -- number of loci controlling quantitative traits, nature of quantitative trait loci, gene action and effects, epistasis, and genotype x environment interaction -- relevant to plant improvement and to molecular marker applications to such improvement are reviewed. Beyond generalities, the conclusion is that quantitative genetics has provided little specific information on the biology or the architecture of quantitative traits. Molecular markers may complement plant breeding in three broad areas. Molecular markers provide reliable estimates of genetic diversity, may improve screening efficiency for many traits through their linkage with alleles with small (quantitative traits) and with large (qualitative traits) effects, and will provide the first understanding of biology and architecture of quantitative traits at the DNA level. Generalities about the usefulness of molecular markers in plant improvement are difficult to make.