The ubiquitous process of globalization is reflected in and affects not only the human economy, but has also given rise to unprecedented environmental changes. The appearance of alien crustaceans is a phenomenon that is very hard to define unequivocally. Newly colonized areas are often inhabited by just a small number of native species, which are susceptible to the effects of human activities. Among invertebrates it is crustaceans that most frequently colonize new water bodies; indeed, freshwater crustaceans are thought to be the most effective invasive non-native species. Species new to the Baltic are often given the negative prefix “xeno” (from the Greek: xenon—foreign), as disruptors of life in that sea. While it is true that most non-indigenous crustaceans have an adverse impact on the newly-colonized environment and that their colonization is invasive, some of them eventually become a regular feature of the ecosystem and an important element of the food web, while others are economically important but with a potential yet to be fully realised. Most new crustacean species are not limited to a specific region but spread very quickly and inhabit further water bodies.