Despite their captivating peculiarity, the insect order Strepsiptera, commonly known as twisted-winged parasites, remains relatively poorly known. Advancements in molecular methods have provided researchers with new tools to address evolutionary questions about the order previously difficult to resolve due to the challenging morphology of Strepsipterans. Stylops, one of the largest genera of Strepsiptera, has had its fair share of these challenges. Stylops are obligate endoparasites of Andrena mining bees, and taxonomists working with the genus have used species concepts with varying degrees of host specificity. This has led to conflicting species hypotheses and unresolved phylogenetic relationships. Furthermore, key evolutionary processes, such as host coevolution and drivers of speciation, have largely remained unexplored. In this thesis, I use genomic-scale data to both clarify the taxonomy of Stylops and to investigate the evolutionary history and processes within the genus. In chapter I, we apply whole-genome sequencing to produce a comprehensive molecular data set focused on West Palaearctic Stylops species. Over 2000 genes were used in species delimitation analyses to evaluate existing and conflicting species hypotheses. We found justification for synonymization of multiple species, indications of undescribed species, and confirmed new host-parasite relationships, clarifying the West Palaearctic species diversity of Stylops. In chapter II, we infer a phylogeny of the genus based on thousands of loci, including newly sequenced Nearctic species. Additionally, we assess the biogeographic history and coevolutionary patterns with their host. The used inference methods produced fully resolved phylogenies with high support values and only minor topological differences between the methods. We found Stylops to originate in the West Palaearctic or West Palaearctic and Nearctic in the early Neogene or late Palaeogene and diversification of Stylops to be shaped by both coevolution and host-shifting. In chapter III we investigate what drives divergence in six species or species groups of Stylops using a genome-wide SNP dataset. We found that, depending on the species, both geographic distance and host association each play important roles for genetic differentiation, highlighting how contrasting processes may shape speciation even in closely related species. In chapter IV, we review the Nordic Strepsiptera fauna based on the current taxonomic knowledge and describe two new Stylops species that were identified as undescribed species in earlier chapters. Additionally, we gather an extensive species occurrence record set of Strepsiptera from the Nordic countries based on museum and private collections, and online databases. The overall aim of this thesis is to apply genomic data to answer questions of species diversity, host association, drivers of speciation and evolutionary history of the genus Stylops to increase our knowledge and understanding of these fascinating insects.