After habitat destruction, invasive alien species are the second leading cause of biodiversity loss, particularly in freshwater ecosystems. They also alter the structure and functioning of ecosystems, lead to biotic homogenisation, and eventually threaten human economies and health. This review aims to synthesise some of the existing information about the world distribution, vectors of spread, and impacts of two important components of freshwater ecosystems, crayfish and fishes. Analysis of the available literature shows that crayfish and fish species, once moved outside their native range, are likely to establish self-reproducing populations, spread from the point of introduction and become invasive. Efforts to manage these populations are difficult and expensive, which warrants the provision of effective preventative measures. Unfortunately, the state of our knowledge of the mechanisms in play in crayfish and fish invasions is still limited, which suggests that much greater attention and investment should be directed to studies in this field.