This chapter aims to cover surgical physiology of the ear and discusses on auditory system and vestibular system. Transduction is a process by which variations in a physical quantity are converted into an electrical signal or vice versa. The elaborate mechanism of the external and middle ear conducts sound from the environment and into the inner ear. Failure of this mechanism produces a conductive hearing loss. In this chapter, the endolymphatic sac (ELS) will be discussed in the section of the physiology of the vestibular apparatus, although dysfunction of the ELS affects the entire inner ear including the cochlea. The vestibular apparatus detects linear acceleration and rotatory motion, plus the direction of gravity. Like the cochlea, it detects mechanical stimuli and transforms them into a change in neural activity. The vestibular apparatus will be discussed in this chapter under the headings endolymphatic fluid and hair cells, otolithic organs, and semicircular canal system. The vestibular neurons, the vestibular-ocular reflex, and other central connections will be also discussed in this chapter.