The invasion of species outside their native range is increasingly frequent and can have serious impact on biodiversity and cause health risks and economical damage to agriculture and forestry. The rapid spread of these alien invasive species has mainly been investigated from an ecological point of view. Little is known about the evolutionary aspects of invasions. lnvasions are rapid evolutionary events in which populations are unusually subjected to founder effects during colonization events followed by a rapid expansion. Thus newly arrived populations are likely to be less variable than the original population from which they derived. This represents a paradox since successful invading species are likely to be genetically depauperate compared to their source populations. We study this paradox using Cameraria ohridella, an invasive microlepidoptera, whose origin is unknown as a model system. Since its discovery in Macedonia, twenty years ago, this insect has progressively colonized all western and central Europe. Here we use molecular markers to study the genetic variability of c. ohridella, across its geographical distribution. The main objectives of this study are (i) to study the genetic structure of both the area of origin and invaded areas of c. ohridella; (ii) to see whether the invasion process and associated demographic expansion is due to changes in the genetic structure of the populations; (iii) to assess whether both invasive insects show a loss of genetic diversity associated with the process of invasion and for the presence of "invasive" haplotypes.