The current degree of adventive species naturalization of the Kaluga Region flora is analysed. The behaviour of the adventive plants is described using 10 grades of the naturalization degree: from the species not able to propagate, to the species settled in natural habitats and capable to transform to some extent plant communities. Main features of each grade are resulted, and the species are specified according to their role in a vegetation cover of the area. The invasive plants species widely extended now in the Kaluga Region are discussed in detail. Their life span, ways of introduction and range of transfer are characterized. Habitats especially liable to introduction of adventive plants are revealed in the region. The speed of dispersal of some naturalized plants and their influence on the indigenous species number in the same habitats are discussed.