Inclusion Criteria:  Adults 18 years of age or older. Candidate for unilateral cochlear implant with Slim Modiolar electrode array as determined by the implanting centre. English spoken as a primary language. Willingness to comply with all investigational requirements. Willing and able to provide written informed consent.  Exclusion Criteria:  Previous or existing cochlear-implant recipient. Evidence of severe or greater sensorineural hearing loss prior to five years of age. Open-set pre-operative word score > 70% in the contralateral ear. Duration of severe to profound hearing loss > 20 years in the ear to be implanted. Adults with functional acoustic hearing in the ear to be implanted who desire to use an acoustic component in the implanted ear. Ossification or other cochlear anomaly that might prevent complete insertion of the electrode array. Hearing loss of neural or central origin. Medical or psychological conditions that would contraindicate undergoing surgery. Women who are pregnant. Additional handicaps that would prevent or restrict participation in the audiological evaluations. Unrealistic expectations on the part of the subject regarding the possible benefits, risks, and limitations that are inherent to the surgical procedure and prosthetic device. Inability or unwillingness to use a hand-held device to control the sound processor and to collect test and survey data. Unable or unwilling to comply with the requirements of the clinical investigation as determined by the Investigator. Investigator site personnel directly affiliated with this study and/or their immediate families; immediate family is defined as a spouse, parent, child, or sibling. Cochlear employees or employees of Contract Research Organisations or contractors engaged by Cochlear for the purposes of this investigation. Currently participating, or participated within the last 30 days, in another interventional clinical investigation/trial involving an investigational drug or device.