Plant species that invade an alien area and outgrow the native vegetation, establishing and increasing their own territory, often lead to negative economic, environmental, and social impacts. Even native species can behave like invasive species by their exponential spread. Similarly, not all non-native species are invasive. Many alien invasive species, however, do threaten the health and integrity of our terrestrial and aquatic ecosystems. As the human population explodes and trade becomes increasingly globalized, the transboundary movement of species from their places of origin to alien regions is escalating and is expected to continue so in the coming decades. The ecological impacts of invasive species, in particular alien plants, on forest ecosystems have attracted the attention of researchers, managers, and policy makers the world over. Alien invaders are known to change the ecosystem structure and function, thereby minimizing the support for native flora, fauna and the provision of ecosystem services that directly benefit humans. There exists a large amount of scattered information on the ecology and management of alien invaders in forest ecosystems. The scientists and practitioners who attended the technical session, "Impact of exotic invasive plant species on forest ecosystems," during the XXIIIUFRO World Congress, Brisbane, 2005, (Ravinder K. Kohli, organizer; Shibu Jose, rapporteur) resolved that an edited volume that captures the current state of knowledge would benefit the global forestry and natural resources community. Invited papers and voluntary posters were presented at this session, which represented a cross section of the current global research being conducted in a variety of forest ecosystems. The editors of this volume accepted the task enthusiastically and immediately started working on the project. Selected authors (who presented their work at the IUFRO Congress) and other prominent researchers were invited to submit manuscripts. After a peer review process, we selected 21 chapters for the current volume.