The forest environment is considered to be more resistant to penetration by alien plant species than other terrestrial environments, which generally are significantly modified by humans. A major environmental threat to vegetation is posed by alien species that are capable of spontaneous expansion in natural or semi-natural communities, and above all in forests. This paper presents a naturalization scale of alien vascular plants in forests of Poland, based on knowledge of species classified as naturalized elements of the Polish flora. Using the research results and observations of other authors, as well as literature data, a list of 180 taxa of such plants was compiled. The classification uses the following categories: life form, systematic affiliation, and origin or status in the Polish flora. The analysis revealed that not all of the considered alien species showed an ability to penetrate forest communities and that only a few of them have developed stable populations in the best-preserved natural and semi-natural phytocoenoses. About 20% of the anthropophytes found in forest communities strongly affect the composition and structure of the forest phytocoenoses.