Nature reserves are often considered to be assemblages of species in natural or semi-natural communities. However, in many parts of the world they also contain exotic species that interact with the native flora and fauna. An International Working Group has been endeavouring to understand the management of invasive species in natural landscapes. Data for four invasive species within the British Isles are analysed. The case studies investigated include Indian balsam (Impatiens glandulifera), rhododendron (Rhododendron ponticum), mink (Mustela vison) and coypu (Myocastor coypus). The rates of spread have been variable, usually increasing after an establishment phase. The discussions concentrate on assessing the impact of invasive species, on deciding whether control measures are feasible and/or desirable, on deciding whether or not nature reserves are less prone to invasion than other habitats, and on assessing wildlife conservation values when invasive species are present.