The muskrat Ondatra zibethicus is one of the semi-aquatic rodents that has been introduced into many areas of the world. It is regarded as a most successful vertebrate invader. In northwestern Europe, it was introduced to Finland in 1919 and to the Kola Peninsula in 1931. From these areas, it spread ‘naturally’ to Sweden and Norway, respectively. Here the history of the introduction, dispersal, population development, impact on plant communities, impact on man, and indirect and direct effects on other animals in northwestern Europe are reviewed. The biological features of the muskrat are discussed in relation to the attributes of other vertebrates that are regarded as successful invaders.