The present thesis analyses the evolutionary history and genetic population structure of the montane caddis fly Drusus discolor. Using a phylogenetic approach, the relationships of several Drusus species and representatives of other genera in the subfamily Drusinae are studied. Sequence data from two mitochondrial loci are used to infer the relationships within and among the genera of the Drusinae. Several hypotheses of relationships previously inferred based on morphological characters were tested. The study revealed a very close relationship between Drusus discolor and D. romanicus. It is likely that divergence between these two species occurred recently. The relationships inferred by molecular data suggest that larval morphology may be an important taxonomic character, which has often been neglected. Two previously unknown larvae from the genus Drusus are identified using sequence data. The population genetic component of this study focuses on the montane caddis fly Drusus discolor. The species is restricted to higher montane regions in Europe and thus shows an insular distribution across all major mountain ranges and highland regions in central and southern Europe. Specimens were studies across the entire species range. mtCOI DNA sequence data from 254 individuals collected from 77 sites were analysed in a population genetic and phylogeographic context using nested clade analysis to infer genetic structure and population history within the species. The data show little molecular variance within populations and regions, but distinct genetic structure between various mountain ranges across Europe. The divergence of lineages was higher than that observed in studies of other caddis flies using the same marker. The pattern is considered strong evidence for the existence of multiple glacial refugia, some of which lie outside of southern Europe. Several refugia and a zone of secondary contact are identified in the region surrounding the Erzgebirge. This study complements theories based on terrestrial species, which claim that glacial refugia were located only in the southern European regions. The study also supports the existence of the biome type “dinodal” for aquatic insects of mountain streams. Possible Pleistocene and post-glacial migration routes of Drusus discolor are proposed.