The genetic basis of complex, ecologically relevant traits is not well known for any organism, despite enormous interest in understanding how such traits evolve. The question is particularly compelling where closely related species have diverged radically in their adaptation to the environment. Differences in host plant use among moths and butteries often provide such cases: Although close relatives tend to use similar hosts, there are many examples of congeneric species that differ widely in host range. In several systems, work is under way to identify the genetic changes that underlie shifts in host use. While such changes may or may not contribute to the well-documented speciosity of phytophagous insects, understanding the genetic architecture of host range is fundamental to understanding the evolution of Lepidoptera. Improved understanding of the genetics of host range is crucial for applied reasons as well: Both the safe practice of biological control and the breeding of plants with persistent resistance to pests demand greater understanding of the genetics of host range. Understanding the evolution of host range in Lepidoptera will require knowledge of its genetic architecture, that is, which genes are involved, how these genes interact, and how much change in each gene is needed for a change in host range.