Tinnitus has many forms; it can be caused by sounds generated in the body (objective tinnitus) that reaches the ear through conduction in body tissue, but much more common is the tinnitus that occurs without any physical sound reaching the ear. Such tinnitus (subjective tinnitus) is a phantom sensation, where abnormal neural activity is generated in the ear, the auditory nerve, or the central nervous system. There are many forms of subjective tinnitus and it can occur with different severity. Subjective tinnitus often occurs in connection with hearing loss such as may occur after exposure to loud sounds (noise), or after administration of drugs such as certain antibiotics, but often no cause can be found. Tinnitus often occurs together with presbycusis and it can occur in deafness. Tinnitus is a part of the symptoms of Ménière's disease and individuals with vestibular Schwannoma almost always have tinnitus. Some individuals who have severe tinnitus hear sounds as distorted and some have hyperacusis (reduced tolerance to sounds) or phonophobia (fear of sounds). Tinnitus can be referred to one ear, or both ears, or to a location inside the head. The anatomical location of the physiological abnormality of chronic subjective tinnitus, however, is rarely in the ear but more often in the auditory nervous system. There are indications that the pathophysiology of unilateral and bilateral tinnitus is different. There is considerable evidence that expression of neural plasticity plays a central role in the development of the abnormalities that cause many forms of chronic subjective tinnitus. Expression of neural plasticity can change the balance between excitation and inhibition in the nervous system, promote hyperactivity, and it can cause reorganization of specific parts of the nervous system or redirection of information to parts of the nervous system not normally involved in processing of sounds (non-classical or extralemniscal pathways). Since there are many kinds of subjective tinnitus, search for a (single) cure for tinnitus is futile. Testing of new treatments is hampered by the fact that it is not possible to distinguish between different forms of tinnitus for which different treatments may be effective.