By definition, parasites are expected to have fitness consequences for their hosts by reducing survival and fecundity. If such events are density dependent they may play a regulatory role in their host's dynamics. However, there are few studies in the wild that provide empirical evidence to support these suppositions. To understand the impact of parasites it is necessary to explore the interactions between parasite and host and the mechanisms that regulate nematode populations. The aims of this work are to: 1) identify the species specific patterns of infection; 2) investigate the interactions between and within nematode species and 3) examine the regulatory mechanisms that control nematode fecundity. The distribution of parasites between hosts and the variation between years, seasons, reindeer age and location are also examined. Nematode infections of Svalbard reindeer are dominated by two species: Ostertagia gruehneri and Marshallagia marshalli and their contrasting life-histories and population dynamics highlights the importance of investigating at the level of individual species. For 0. gruehneri, there is significant annual variation but no strong seasonal pattern in abundance. Susceptible calves do not acquire infection until their second summer. Egg output is highly seasonal with a peak in July and controlled through density dependent effects on worm development. In contrast M marshalli, shows a strong seasonal cycle which does not vary between years and the peak occurs in late winter suggesting winter transmission. Egg output is low and also confined to the winter months. The quantification of these traits is important in allowing parameterisation of models with data from the study system. In many studies parameters are estimated from studies of domestic host parasite systems and these may be inappropriate in this natural system. The role of immunity and arrested development and the relationship between transmission and environmental heterogeneity are discussed.