Even though the diagnosis of otosclerosis is confirmed definitively during surgery, preoperative diagnosis and determination of the indication for surgery are made based on audiological evaluation. Audiological tests should firmly establish a conductive component to hearing loss. The measurement of pure-tone air and bone conduction thresholds has limitations that prevent an accurate diagnosis based solely upon these test results. Such limitations include general variability of threshold measurements, individual variations of tests in the bone conduction mode, and complex interactions between changes of middle ear mechanics and threshold. Objective audiometric tests should be added because of these uncertainties. The presence of otoacoustic emissions effectively excludes a diagnosis of otosclerosis. Standard clinical immittance measurements are used to confirm (or exclude) otosclerosis, the typical pattern being a normally shaped tympanogram and absent stapedial reflexes. Multifrequency tympanometry adds little information. Aside from establishing a preoperative diagnosis, audiological evaluation provides quantification of hearing loss, upon which the indication for surgery is based. Moreover, it lays the foundation for evaluation of surgical success and outcome measures. For both of these aims, speech audiometric tests such as a speech reception threshold should be included in the preoperative audiological evaluation of patients with otosclerosis.