Biological invasions by crustaceans represent a serious threat for native species in Europe. My thesis brings information on non-indegenous freshwater crayfish introduced to Europe and their parasite Aphanomyces astaci, the pathogen of the crayfish plague. Chapters I-V focus on genetic variation in North American crayfish. We showed that in two crayfish specie, both successful invaders in Europe, genetic variation differs significantly, reflecting their different colonization histories on the continent. Orconectes limosus was likely introduced to Europe just once, in small numbers. Variation at the mitochondrial DNA level in Europe is relatively low (chapter I), although some variation was revealed by nuclear markers in its Central European populations (chapter II). In contrast, Pacifastacus leniusculus was introduced to Europe several times, in large numbers. Its European populations are genetically highly diverse (chapter III). Chapter V showed the utility of DNA barcoding for identification of newly established species in Europe. The parasite A. astaci appeared in Europe in 1859 and has substantially reduced native crayfish populations. North American crayfish may carry the pathogen and transmit it to European species. Information on A. astaci in invasive crayfish is therefore important for conservation of native species. In chapter VI, we confirm that P. leniusculus serves as a reservoir of the pathogen in France and represents a serious danger for native species. In chapter VII, we validate that a real-time PCR is suitable for the detection of the pathogen.