As the animal body size and activity levels increased, the circulatory system underwent significant changes that profoundly influenced animal anatomy and physiology. Early life forms lacked complex circulatory structures and relied mainly on simple diffusion and slow bulk flow within rudimentary gastrovascular spaces. Such mechanisms were sufficient only for small organisms with low metabolic demands. With increasing morphological and physiological complexities, animal lineages independently evolved specialized mechanisms for internal fluid transport. Some groups retained relatively loose, open circulatory systems in which fluid circulates through body cavities, whereas others developed closed networks of vessels that confine blood to defined pathways. Circulatory diversity is extensive, ranging from simple tubular hearts in small crustaceans and accessory pumping structures in earthworms to fully developed, complex, multichambered hearts characteristic of vertebrates such as birds and humans. Among vertebrates, circulatory organization gradually progressed from single circuits to partially divided and fully separated systems, ultimately giving rise to the four-chambered hearts observed in mammals and birds. This evolutionary refinement is essential for warm-blooded animals to sustain high activity levels and maintain elevated metabolic performance. Modern circulatory systems consist of more than a central pump and connecting vessels; they include the heart, extensive vascular networks, microscopic capillary beds, and lymphatic drainage, all shaped by evolutionary pressures. Examining how these systems have changed across species helps explain the unique traits of different animals, the structural diversity of their hearts, and the relevance of circulatory features in medical and forensic contexts. Rather than focusing on a single aspect, this chapter integrates evolutionary history, structural variation, and functional organization to illustrate the wide range of circulatory designs and the common principles underlying them.