Cochlear implant surgery is indicated for patients of all ages with bilateral severe or profound sensorineural deafness. To date, the youngest age for cochlear implantation worldwide is 4 months and the oldest age is 97 years. There are two types of deafness: prespeech deafness and post-speech deafness, and the specific selection criteria for cochlear implantation are: [Patients with prespeech deafness] (including infants) ① Patients with prespeech deafness who have severe or profound sensorineural deafness in both ears are suitable for cochlear implantation. ②For prelingually deaf patients with residual hearing, cochlear implants should be fitted as early as possible when there is no significant improvement in auditory speech ability after trial of hearing aids. The best age for cochlear implantation for patients with prespeech deafness is 12 months to 5 years old; since the younger the age of implantation, the better the rehabilitation of hearing and speech, cochlear implantation can be performed at 6-12 months of age in a hospital with good surgical conditions for children with good physical development. For children who have cochlear implants after the age of 3, they need to undergo post-operative hearing and speech rehabilitation training, so they must have the conditions to undergo hearing and speech rehabilitation training before they can undergo cochlear implant surgery. Patients with postlingual deafness] (including senile deafness) ①Patients with postlingual deafness who have severe or very severe sensorineural deafness in both ears can undergo cochlear implantation as long as they are in good health and have no contraindications to surgery. ②Cochlear implantation should be performed in patients with postlingual deafness with residual hearing when hearing aids are ineffective or very ineffective and open utterance cognitive test scores ≤ 30%. It is worth mentioning that in both prelingually and postlingually deaf patients, the ideal auditory outcome after cochlear implantation can only be obtained when the auditory conduction pathway of the auditory nerve up to the auditory center of the cerebral cortex is functioning intact.