Adaptation to different host taxa is a key driver of insect diversification. Herbivorous insects are classic models for ecological and evolutionary research, but it is recent advances in sequencing, statistics, and molecular technologies that have cleared the way for investigations into the proximate genetic mechanisms underlying host shifts. In this review, we discuss how genome-scale data are revealing-at resolutions previously unimaginable-the genetic architecture of host-use traits, the causal loci underlying host shifts, and the predictability of host-use evolution. Collectively, these studies are providing novel insights into longstanding questions about host-use evolution. On the basis of this synthesis, we suggest that different host-use traits are likely to differ in their genetic architecture (number of causal loci and the nature of their genetic correlations) and genetic predictability (extent of gene or mutation reuse), indicating that any conclusions about the causes and consequences of host-use evolution will depend heavily on which host-use traits are investigated. To draw robust conclusions and identify general patterns in host-use evolution, we argue that investigation of diverse host-use traits and identification of causal genes and mutations should be the top priorities for future studies on the evolutionary genetics of host shifts.