The book examines the tremendous development and progress of zoology in the 20th century. It traces the most important achievements in many branches of the vast field of zoological science—systematics, morphology, physiology, ecology, zoopsychology, ethology, hydrobiology, and zoogeography. Along with the most remarkable discoveries in various fields, information is provided about the scientists who made them. Based on numerous facts, a comprehensive picture is obtained of the epoch-making achievements of zoology in the 20th century, including the sensational discoveries of some new, previously unknown species of animals, as well as the establishment of entire types and classes that were unknown to zoologists. The book also reflects some of the most significant achievements of Bulgarian zoologists in the 20th century.