Alternative hearing aid amplification and signal processing schemes have been developed (a) to suppress noise, improving listeners' subjective assessments of the resulting sound quality and/or (b) to improve speech intelligibility for hearing-impaired listeners. This paper presents selected studies of the effects of various types of processing on both perceived quality and measured intelligibility. One study examined the effects of low- and high-cut filtering, superimposed on a suitable hearing aid gain function, using simulations running on a digital-signal processing (DSP) board. Another study compared two types of amplification systems, using purpose-built hearing aids in which the gain functions had been precisely matched to the needs of the individual hearing-impaired listener.