Crayfish represent an important element of freshwater ecosystems. However, there has been a drastic decrease in the number of autochthonous species of crayfish in Europe over the past 200 years. Epidemic outbreaks of crayfish plague caused by the pathogen Aphanomyces astaci have been reported as one of the main causes of this decrease in abundance. The infective units of the disease are secondary zoospores produced at hypha ends of this obligatory parasite. Apart from crayfish, other crustacean species have also been identified as pathogen’s hosts and carriers. There are five described genotype groups, two of which have multiple subtypes. It is considered that the all of the strains originate from two waves of introduction of North American crayfish species into Europe. Multiple types of horizontal transmission of A. astaci have been observed, such as by fish, birds and anthropogenic activities (fishing equipment, ships and pet trade). Latent infections have been discovered in the Balkan region, which are considered to have been caused by the strains introduced by the first two phases /waves. Subtype differences within the different strains of A. astaci and their effect on ecosystems in which they are present are still inadequately studied. Furthermore, additional research of infection control methods of this pathogen is required to reduce the impact of A. astaci on European freshwater crayfish and ecosystems in general.