The sheep tick Ixodes ricinus (L.) is known to transmit a large number of pathogens of medical and veterinary importance, including the bacterium Borrelia burgdorferi sensu lato, the causative agent of Lyme borreliosis. Ixodes ricinus is found predominantly in woodlands, which provide suitable hosts and microclimates for tick survival. Three blood meals from a wide array of possible hosts are needed to complete the tick life cycle within 2 to 6 years. Ticks can acquire B. burgdorferi s.l. during a blood meal on an infected host and can infect a new host during the next blood meal. In this thesis, interactions between hosts, habitat characteristics, and microorganisms in relation to the biology of I. ricinus were investigated. A longitudinal study that was initiated at 24 sites across The Netherlands revealed that ticks infested with B. burgdorferi s.l. were found at all sites, but with strong spatial and temporal variation. This variation could be partially attributed to habitat characteristics. Substantial variations in the bacterial diversity within I. ricinus nymphs were also observed between habitats. Molecular genetic analyses indicated the presence of several potentially pathogenic and non-pathogenic bacteria. The interactions of these bacteria with hosts, ticks and other micro-organisms that reside in the tick need to be established in the future. It is known that rodent populations can mediate B. burgdorferi s.l. circulation in nature. Therefore, we studied the potential effects of rodents on the spatial variation of Borrelia infections in host-seeking ticks. Wood mice (Apodemus sylvaticus) and bank voles (Myodes glareolus) were abundant at most sites, but had varying tick burdens and Borrelia-infection prevalence between sites. It is argued that the infection prevalence in host-seeking I. ricinus ticks in the vegetation depends on the degree of contact between ticks and mice, which is, in turn regulated by habitat characteristics.Larger hosts, such as ruminants, are important for tick reproduction. ...