Objective: This systematic review evaluated the effectiveness of two frequency-lowering schemes, non-linear frequency compression and frequency transposition, at improving speech intelligibility for adult hearing-impaired populations. Design: A systematic search of 10 electronic databases was carried out using pre-defined inclusion criteria. Accepted articles were then critically appraised using the Effective Public Health Practice Project (EPHPP) Critical Appraisal Tool. Outcome results were further synthesised where possible using random effects meta-analysis to provide overall combined estimates of the treatment differences along with 95% confidence intervals. Study sample: A total of 20 articles were accepted for final review. Results: Overall, study quality was of moderate strength. Meta-analysis found a statistically significant benefit in favour of frequency-lowering for consonant recognition testing in quiet across 145 participants with both algorithms providing comparable gains. Equivalent results were found between frequency-lowering and conventional processing on all other speech measures. Conclusions: Based on the available data, frequency-lowering does seem to provide some improvement in an individual’s speech intelligibility dependent on the stimulus type, although the benefits were modest. This improvement was not seen across all measures, however those who do not benefit from the technology will also not be harmed by trialling it.