The tendon-sheath mechanism is an important method of mechanical transmission. Different from gears, worm gears and other transmission methods, tendon-sheath transmission has outstanding advantages like lightness and flexible transmission path. Due to these benefits, it has been widely used in various robots, including slender robots, medical robots, exoskeleton robots, etc. In this chapter, we discuss slender robots and delve into how tendon-sheath transmission is applied in these systems. Additionally, the current research of tendon-sheath transmission theory will also be discussed. Moreover, the bionic prospect of tendon-sheath design is also explored with the inspiration of skeletal muscles. This chapter will help readers to understand the tendon-sheath transmission and its main robot application status.