Plant-insect interactions are a wide and diverse field of study. Abiotic and biotic factors influence the dynamic interaction between plants and their insect herbivores. Lymantria dispar (gypsy moth), Lymantria monacha (nun moth) and Tortrix viridana (green leaf oak roller) are important forest pest species. Not much is known about the extent of genetic diversity within and among these insect populations in comparison to the respective host trees. The main focus of this thesis is the genetic basis of adaptation of tree-feeding insect larvae in the genus Lymantria and Tortrix to their host trees. Gene expression at the mRNA level of insect larvae feeding on different tree species was analysed, and similarly gene expression in the host trees undergoing herbivory in the field was measured. Gene diversity at the genomic DNA level was assessed in a population genetic study of one Lymantria species. As part of an EU project (EVOLTREE - Evolution of Trees as Drivers of Terrestrial Biodiversity) patterns of adaptation were evaluated by using different molecular and ecological techniques. In general a diverse mixture of techniques and methodologies was used to generate new genetic information on plant-insect interactions. Digestive adaptations in the insects were identified from the herbivory laboratory studies and the host plant responses were identified in the field study. The cDNA-AFLP technique enabled the combination and challenge of field and laboratory experiments to be met and produced a detailed overview in respect of defense strategies on both sides of the interaction.