Alien -- also called non-indigenous or non-native -- species are defined as those species that colonise an area beyond their natural range, where they reproduce and establish a population. It is known that plants, animals and micro-organisms have been intentionally transferred with human discovery voyages for centuries and, possibly, since the shift from foraging to agricultural and pastoralist societies. As a result, many 'exotic' species are now among our preferred foods, dearest pets, good-looking houseplants and decorative aquarium weeds, but most of them cannot be considered alien species, as they grow well only in artificially-controlled conditions or in protected environments. Conversely, alien species are capable of ready acclimation to novel habitats, where they may find themselves unconstrained by the limiting factors -- both abiotic and biotic -- typical of their original habitat. In these 'favorable' conditions, they might outgrow, and ultimately overthrow, resident organisms with which they happen to compete: in this case, alien species are often dubbed 'invasive'. This book discusses patterns, management and economic impact of these biological invasions.