In this study, there are given descriptions and reasons for the endangerment of the most endangered animal taxa in the Republic of Croatia according to the Red List. The Red List is one of the key professional bases in nature protection. It contains a list of species or subspecies of plants, fungi and animals that are in potential danger of extinction and the degree of the danger of their extinction. In this paper, attention is paid to the fauna, i.e. endangered fauna in the Republic of Croatia: mammals, birds, amphibians, reptiles, freshwater and marine fish and diurnal butterflies. There are many reasons for the disappearance of mammals in Croatia. The biggest reason is overhunting, and far from overhunting is neither poaching nor intensive agriculture with mechanization that leaves no room to preserve the fauna. According to European standards, the Croatian ornithofauna is very rich and diverse, however, out of 231 nesting species, as many as 78 belong to endangered species. One of the most important roles in the world's ecosystem is played by amphibians and reptiles, they are a necessary part of the food chain. The group of vertebrates that is the most diverse and most numerous, and the least researched are precisely the fish. Fish live in salt and freshwater, of which one third belong to freshwater taxa. How freshwater ecosystems are sensitive to the pressure to which they are exposed by human action puts fish in an unenviable position. In addition to being the most diverse, fish are also the most endangered animal groups in the sea. The Red Book contains a list of 123 species and subspecies of fish, and more than 75% of them are classified in one of the IUCN endangered categories. Scientists pay special attention to day butterflies. Their numbers in Europe have decreased significantly, so various conservation measures for these species have been proposed. In 2004, the Red List of Endangered Plants and Animals of Croatia was compiled and included 38 species of the most endangered butterflies.