Prescriptive fitting formulae have been used with linear hearing aids for many years. Especially the NAL-R formula has been accepted and used throughout the world. While the application of generic fitting rules like the NAL-R rule is likely to result in acceptable fittings for linear hearing aids, the use of generic fitting rules for multi-band non-linear hearing aids is more problematic. This is especially true when these hearing aids include one or more functions not related to the hearing loss, for instance adaptive systems for noise reduction or adaptive time constants for the compression function. Problems arise because a large number of factors related to the construction of the hearing aid significantly influence the sound reproduction. In this presentation, the effects of such factors are discussed. Thus, the influence of filter bandwidth, number of channels, detector type, compression threshold, attack and release times, and distortion on sound reproduction will be discussed. Moreover, certain hearing aid functions adapt to the listening environment and these are, in general, device specific and may further complicate the use of general fitting rules. These observations have made most hearing aid manufacturers develop hearing aid specific fitting rules. These may be beneficial when adjusting a specific hearing aid, but as Byrne (1996) pointed out, they are typically proprietary and their effect is likely to be insufficiently documented. Various possible ways to unite the two fitting strategies (generic vs. device specific) are presented.