Abstract Objectives: The aim of this study was to seek a greater understanding whether it is beneficial to implant pre- and postlingually deafened adults, and if it is beneficial to implant unilateral CI users with a second implant. Study design: Review of the literature Methods: Published literature on the topic of prelingual and postlingual deafness among adults was reviewed, as well as literature on the topic concerning additional benefits from a second CI. A total of 24 articles were included, and the content was systematically organized in tables. Results: Oral communication and the use of hearing aids prior to implantation, are good prognostic factors for CI outcome in prelingually deafened adults. Prelingual deafness should not be an exclusion criterion for CI. No upper age limit were found, where a CI did not have effect in the postlingually deafened adults. An increasing age might be associated with poorer test performance in noisy environment. When receiving a second CI, a long inter-implantation delay should not be a contraindication, but a short inter-implantation interval is desirable. The inclusion criteria for CI recipients should be broader, and evaluated on a case-to-case basis. Conclusion: CI is beneficial in all three examined patient groups.