Inclusion Criteria:  Adults >18 years old. Fluent in Danish, including reading and writing Willing to participate in and to comply with all requirements of the protocol. Post-lingual deafness and use one or two HAs. Participants should have aidable hearing in the ear not considered for CI implantation Self-reported HA use of at least eight hour per day for at least one year in both ears prior to evaluation for cochlear implantation in order to ensure, that both ears have received auditive stimulation prior to participation in the study. If possible, the validity of the self-reported hearing aid use will be checked in the HA-log by the audiology assistants. Participants should have a PTA (0.5,1,2,4 kHz) > 40 dB HL in the ear considered for CI implantation and PTA≥40 and ≤ 70dB HL in the ear not considered for implantation. A speech identification score <50 % in the poorer hearing ear, i.e., the ear considered for implantation, and <70% in the best-aided condition is required.  Exclusion Criteria:  Unwilling or unable to comply with investigational requirements. Normal hearing in the better hearing ear (single-sided deafness). Not using HAs; long-term, un-stimulated deafness defined as no acoustic stimulation (aided or normal) in one or both ears greater than one year, which will be judged based on patient interviews and through hearing aid log data. Vestibular loss in the ear not considered for CI implantation, which will be determined by the video Head Impulse Test (vHIT) prior to implantation. Surgical issues interfering with the site of implantation or anatomical contraindications such as cochlear malformations. Medical contraindications such as chronic middle ear disease. Tympanic membrane perforations. Auditory nerve lesions. Central auditory pathway pathologies. Otosclerosis.