The young field of invasion biology - initially a branch of ecology and conservation biology - has greatly expanded, particularly in the last two and a half decades or so. As a result, the potential negative effects of introduced species have been widely advertised and sometimes, perhaps, overemphasized. This book attempts to restore some balance to the current debate over the role of non-native species, by offering a broader perspective, and taking a longer term, evolutionary look at these species and their impact in their new environments. The relatively arbitrary nature of terms such as "native" and "non-native", and the rather inconsistent ways in which such terms are applied to biological species, as well as the subjective boundaries of so-called "native ranges" are analyzed. The role of non-native species in their new environments can be considerably more complex than the anti-introduced species information would often suggest. Thus, the more positive and nuanced perspective on introduced species and their impact offered in this book is much needed and long overdue.