Third window phenomena result when there is an abnormal (or abnormally large) connection between the labyrinth and some other space. Such abnormal connections can be congenital or acquired. A third window reduces the hydraulic resistance in the labyrinth, making it easier for fluid waves propagating through the labyrinth to stimulate cochlear and vestibular hair cells. This leads to clinical manifestations such as hyperacusis, sound/vibration-induced oscillopsia, nystagmus, and disequilibrium. True third window phenomena include semicircular canal dehiscence, carotid dehiscence, and temporal bone fracture involving the labyrinth; other conditions that reduce the labyrinth’s hydraulic resistance include large vestibular aqueduct and large cochlear aqueduct. These conditions can often be diagnosed on temporal bone CT. Treatment is surgical.