Cochlear Implants are a well-established method to enable deaf or nearly deaf patients to recover hearing capabilities through electrical stimulation of the auditory nerve. Most cochlear implant users are able to reach a very high level of speech understanding in quiet, are able to communicate via telephone and watch television or cinema. However, they still report problems with speech understanding in noisy environments and with music perception. Furthermore, the clinical every day situation shows that although the indication for a cochlear implant has been given, there are some patients with more problems than others getting used to the cochlear implant. In this work the years of experience with fitting of cochlear implant speech coding strategies to individual preferences were used to investigate the effects of some key parameters of fitting. In the first part of the work the effect of the thickness of the skin flap is investigated. During cochlear implant surgery there are several habits on how to deal with the skin flap. While some surgeons are carefully thinning the flap, others do not. During daily life of a cochlear implant user, the thickness of the skin flap may have effects on the stimulation parameters, the speech perception and the battery life of the speech processor. Ways to determine the thickness of the skin flap and its effect on some of these parameters are investigated. In the second part, the recent development of speech coding strategies towards frequency specific stimulation of electrodes is further investigated by performing a pitch perception test. The so-called fine structure (FS) speech coding strategy by the cochlear implant company Med-El was compared with the former continuous interleaved sampling (CIS) strategy when only two or three electrodes were activated, and the pitch of sinusoidal stimuli had to be discriminated. The effects of the number of active electrodes and of the stimulation rate were investigated. In the third part of the work, the effect of modifying the filter mechanism of the filter bank was studied by comparing the normally used Finite Impulse Response (FIR) filters, and modified FIR filters. The work shows that most tested parameters have effect on speech perception, melody recognition, or pitch perception. Therefore, clinicians doing cochlear implant fittings must be aware of those parameters and understand their effects.