Rapid urea permeation mediated by urea transporters (UTs) is crucial for maintaining normal physiological processes in organisms. UTs not only facilitate urea transport but also water transport, further underscoring their role in maintaining fluid balance. Advances in structural biology have led to the elucidation of high-resolution three-dimensional structures of various UTs, offering critical insights into the molecular mechanisms underlying their efficiency in transporting urea and water. UT-B displays high permeability to urea analogs, which can competitively inhibit urea permeation by obstructing the channel. However, whether UT-A is capable of transporting urea analogs remains contentious. Additionally, further investigation is required to determine if UTs can facilitate ammonia transport. Urea permeability (Purea) in erythrocytes differs between different mammals. Carnivores exhibit high Purea. In contrast, herbivores show much lower Purea. Erythrocyte Purea in omnivores was intermediate. Rodents and lagomorphs have Purea intermediate between carnivores and omnivores. This chapter provides information about the transporter characteristics of UTs.