As the tendon-sheath system is compliant and has low stiffness, control of these tendon-sheath systems could be challenging. On one hand, they have a lot of nonlinear factors including the compliance and friction between the tendon and sheath; on the other hand, due to spatial restrictions and disinfection requirements, these systems usually lack corresponding sensory feedback. These challenges limit the application of tendon-sheath systems in situations requiring precise control. To solve the problems, many studies on modeling and analysis of tendon-sheath transmission characteristics are reported. This chapter introduces the transmission characteristics of both single tendon-sheath and double tendon-sheath mechanisms, providing a foundation for modeling other tendon-sheath systems. To aid in the design and optimization of tendon-sheath systems, the chapter analyzes the main factors influencing tendon-sheath transmission. Furthermore, the most important factor, the bending curvature of sheath is discussed and a curvature optimization method is introduced.