Astacus (from the Greek αστακός, astacós, meaning "lobster" or "crayfish") is a genus of crayfish found in Europe and western Asia, comprising three extant and four extinct, fossil species. Due to the crayfish plague, crayfish of this genus have declined in Europe and have in many European regions been replaced by the North American signal crayfish, which carries the plague but is unaffected by it.