The paper gives an overview of non-native and invasive tree species, and offers the basic data about their current presence in European forests. It deals with definitions and classifications and explains how these species can affect new environment with the emphasis on local biodiversity loss. The process of invasion is also explained, emphasizing that there is a lag phase in which a species does not reveal its invasiveness, which can last even a few centuries (for certain species). That is why it is important to monitor non-native species and prevent their further establishment. Invasive species are less likely to develop in habitats with intense inter-species competition over a long period of time, in undisturbed areas with high biodiversity, but also in areas with the lack of nutrients, light and water or in areas where these resources are being heavily exploited by local plant communities. nema