For people with severe to profound sensorineural hearing loss, a cochlear implant (CI) is a valuable treatment that improves auditory functioning and speech perception in a quiet environment. However, speech perception in background noise is challenging or even impossible for CI recipients. This thesis focused on: 1) CI listeners’ speech perception in background noise in terms of speech recognition, noise tolerance, and listening effort; 2) the effect of clinically available single-microphone noise reduction algorithms on these outcome measures; 3) the relative contribution of the available speech cues in the incoming signal and cognitive top-down processing to speech perception in noise; 4) improvements in methods to test speech perception in noise.