This chapter discusses orthodromic and antidromic nerve conduction studies. Nerve conduction studies (NCS) typically involve mixed nerves, such that they include both sensory and motor nerve fibers. Motor nerve action potentials propagate from the anterior horn cell distally toward the neuromuscular junction. Sensory nerve action potentials propagate from the sensory nerve endings proximally toward the dorsal root ganglion. Propagation of an induced action potential during NCS travels in both directions, proximally and distally. In an orthodromic study, the recording electrodes measure the action potential traveling in the physiologic direction. In an antidromic study, the recording electrodes measure the action potential traveling opposite the physiologic direction. These orthodromic studies measure the compound motor action potential in the physiologic direction, such that the stimulation site is proximal and the recording electrode is placed distally over the muscle belly. The chapter discusses motor nerve conduction studies and sensory nerve conduction studies.