Tinnitus is not a disease, but a symptom that can result from a wide range of underlying causes: abnormally loud sounds in the ear canal for even the briefest period (but usually with some duration), ear infections, foreign objects in the ear, nasal allergies that prevent (or induce) fluid drain, or wax build-up. Withdrawal from a benzodiazepine addiction may cause tinnitus as well. In-ear headphones, whose sound enters directly into the ear canal without any opportunity to be deflected or absorbed elsewhere, is a common cause of tinnitus when volume is set beyond moderate levels.

Tinnitus can also be caused by natural hearing impairment (as is often seen in aging), as a side effect of some medications, and as a side effect of genetic (congenital) hearing loss. However, the most common cause is noise-induced hearing loss.

As tinnitus is usually a false subjective phenomenon, it is difficult to measure using objective tests, such as by comparison with noise of known frequency and intensity, as in an audiometric test. The condition is often rated clinically on a simple scale from "slight" to "catastrophic" according to the practical difficulties it imposes, such as interference with sleep, quiet activities, and normal daily activities.