The hard tick Ixodes ricinus is one of the most important species of disease vectors worldwide. It transmits a variety of pathogens, including spirochaetes that are the causative agent of Lyme borreliosis (LB) in humans. This study aimed at analysing different aspects of the ecology, molecular evolution, and microbial associations of I. ricinus. A novel scheme for the phylogenetic analysis and genotyping of I. ricinus was established as part of this study. Phylogeographic clustering of I. ricinus samples fromdifferent European countries was observed, while samples from two different sites inSouthwest England did not show spatial differentiation. The ecology and host abundance in these two habitats was assessed. Differences in the density of questing ticks and in the abundance of rodent hosts were observed. Both sites exhibited low densities of rodents and of ticks infesting them. A blood meal analysis revealed high proportions of mixed feedings and showed artiodactyls to be a main host group for immature I. ricinus ticks from Britain. The prevalence rates of Wolbachia endobacteria in British I. ricinus samples were higher than rates found in samples from other European countries. The unique endosymbiont Midichloria was detected in all female I. ricinus ticks that were screened. A newly developed multi gene analysis of Midichloria samples from six European countries showed a largely coordinated phylogeography with their tick hosts. Taken together, the findings of this thesis demonstrate that British I. ricinus ticks differ from their European counterparts in several ways, which has implications for the epidemiology of infectious diseases transmitted by this vector.