This chapter discusses otospongiosis, which is a primary disease of bony labyrinth in which irregular spongy bone replaces the dense endochondral layer of bony otic capsule. In clinical otosclerosis, the symptoms of hearing impairment are present. Malignant otosclerosis is a severe type of cochlear otosclerosis. Fissula antefenestrum and fissula postfenestrum and otic duct are the appendages of otic capsule. In stapedial otosclerosis, the lesion starts at the site of predilection which may be at fissula antefenestrum, fissula postfenestrum, circumferential, biscuit focus or obliterative focus. In cochlear otosclerosis, round window and other areas of the capsule are involved. Tympanic membrane is normal and mobile. Dangers of fluoride therapy are fracture long bones and spine due to fluorosis. X-ray spine and long bone to see the thickening of trabeculae should be done as a routine. Hearing aid is the best for those patients who do not want or are unfit for surgery.