Cestodes (tapeworms) are among the most widespread endoparasites affecting freshwater and marine fish worldwide. Their complex life cycles involve multiple hosts and ecological interactions that facilitate transmission through aquatic food webs. Fish may serve as either intermediate or definitive hosts depending on the cestode species. Understanding the relationships between definitive hosts, intermediate hosts, and environmental factors is essential for fish health management, biodiversity conservation, and aquaculture development. This article examines the principal cestode species found in fish, identifies their main and intermediate hosts, and analyzes the natural pathways of transmission and circulation in aquatic ecosystems.