An overview of the most common diseases of native and naturalized crayfish in Europe is presented. It mainly focuses on findings of the last decade covering viruses, bacteria, protozoans, fungi, DRIPs, and branchiobdellids. It further addresses the problem of interspecific transmission of crayfish diseases in the wild and presents some examples of pathogen transfer from introduced to native crayfish species and vice versa. These examples demonstrate that transmission of pathogens among crayfish species is not only a theoretical threat but has repeatedly happened in European waters. Particularly the documented introduction of different genotypes of the crayfish plague fungus, Aphanomyces astaci, together with their American host species Pacifastacus leniusculus and Procambarus clarkii into Europe and the proven transmission of at least one of these genotypes to native crayfish underlines the risk of introduction of alien crayfish species for our indigenous fauna.