The challenge posed by multifactorial genetics is to understand the genetic contribution to any phenotype that results from the action of multiple genes and the environment. The phenotypes are as diverse as common human disease and crop yield, and they have become the focus of attention for scientists with a broad range of backgrounds, interests and expertise. This convergence of disciplines is illustrated by four reviews in this issue, which cover the statistical methodology that underpins the search for quantitative trait loci, how complex trait genetics can be used to improve domesticated species, the evolutionary forces that maintain quantitative genetic variation, and the physiological tools available in the rat to model human complex traits. On our web site, we have collected these articles in a Web Focus, along with other relevant reviews that we have published over the past year. These include articles that discuss the design of association studies in humans, the crucial lessons that have been learned about multifactorial traits from studies in Drosophila, and the strategies that can be used to move from a mapped locus to the causative molecular variant in mice. To complete the collection we have added some recent papers that report key research findings published by other journals from the Nature Publishing Group. The intense interest in multifactorial genetics reflects the huge potential for rewards: benefits for human health and productivity; a better understanding of evolutionary processes; and fuller knowledge of the connections between genotype and phenotype. The scale of the problem should not be underestimated, but with the current convergence of minds, resources and ideas we can afford to be optimistic.