The winter moth is a insect with a very broad host range. In the last two decades, this species has expanded its range further and has been observed at outbreaked densities on many host plants. This thesis adopts an evolutionary perspective on these changes in host use, in order to determine the role of the host plant in the mediation of genetic change. A panel of molecular genetic markers were developed to investigate the population genetics of numerous host associated populations. The presence of host-associated genetic variation in winter moth populations was observed and the combination of population genetic data allowed the detection of genetic variation over different temporal and spatial scales. A number of host-specific mitochondrial DNA lineages were detected and were found to be largely present in either heather or deciduous feeding populations, but rarely in both, suggesting little genome introgression. The inference of population bottlenecks in the host-range expansion of this species was also possible. In many cases, the levels of population subdivision were high and statistically significant. Inferred levels of gene flow were also sufficiently small to bring about population differentiation. Much of the observed genetic differentiation was present over small spatial scales, in areas of natural sympatry and therefore provided evidence for the evolution of host-associations in this species. The results of this investigation are discussed in the light of the current ecological evidence for resource-associated differentiation in the winter moth. It is argued that host-associated population differentiation has occurred, both in areas of allopatry and sympatry. The occurrence of areas of sympatric differentiation are of great evolutionary importance in the speciation process.