The monograph includes a description of 100 freshwater crustaceans from the order Harpacticoida (Copepoda) from the surface and underground waters of Bulgaria. The described species belong to 14 families, 18 genera, and 8 subgenera. Each species is described in terms of important taxonomic features, bionomics, habitats, and individual variability. For each species, a description of important taxonomic features, bionomics, habitats, and individual variability is provided. Issues concerning the origin and evolution, systematics, and biology of the modern harpacticoid fauna are discussed in detail. The geographical location and zoogeographical distribution of the harpacticoid fauna of Bulgaria at the genus and species level are presented. Information is provided on the horizontal and vertical distribution of species and their bionomics in relation to their biotope. Special attention is paid to the origin and species composition of the Bulgarian subterranean harpacticoids and a comparison is made with the harpacticoids from other Balkan countries. A key for determining families, genera, and species has been developed. The description of the species is supported by detailed illustrations of the individual morphological features that are important for taxonomy.