Interest in the culture of freshwater crayfish throughout Europe has increased during the last few decades. However, the effects of both introduced and native crayfish species on the structure and diversity of freshwater communities has received little attention. In this paper, I review the impact of crayfish on macrophytes, algae, invertebrates, amphibians and fish; also I compare the impact of introduced and native crayfish species. By feeding selectively on submerged species and seedling stages of emergent macrophytes, abundant crayfish populations reduce the biomass and species richness of such macrophytes. The impact of crayfish on algae is complex and less predictable: macro-algae may be grazed by crayfish, whereas micro-algae may increase when crayfish reduce the number of invertebrate grazers. Among the invertebrates, thin-shelled snails are particularly vulnerable to crayfish predation, whereas swimming invertebrates are affected less. While crayfish consume amphibian eggs, larvae, and froglets, the impact, however, of crayfish on amphibian eggs deposited when water temperature is low is probably negligible. Amphibian larvae may out-swim crayfish, whereas less mobile froglets will be vulnerable to predation. Some amphibians such as bufonids may be unpalatable to crayfish (e.g. Pacifastacus leniusculus and Astacus astacus), Procambarus clarkii being a possible exception. The loss of vegetation in crayfish-rich environment has several indirect effects on biodiversity and may reduce the value of a habitat for invertebrates, amphibians and trout fry. Bird species dependent on macrophytes and invertebrates as food may be negatively affected, whereas birds utilizing crayfish as food may benefit from their introduction. Most crayfish species have similar preferences for macrophytes and invertebrates, but due to greater consumption and more rapid population development the introduced species may have stronger impact on macrophyte and invertebrate biomass. Moreover, these introduced species appear to have adaptations that differ from native species. Procambarus clarkii may, for example, colonize ephemeral habitats that are uninhabitable to native crayfish.