The aim of this thesis was to improve understanding of the conditions required to measure acclimatisation. This should result in a more robust methodological framework for measuring acclimatisation in future studies. Three experiments were undertaken on separate groups of 16 subjects with moderate bilateral sensorineural hearing impairments. Subjects were fitted monaurally with a linear hearing instrument that provided approximately 20 dB real ear insertion gain at 2-4 kHz. The not-fitted ear was used as the control. The self-reported use of the hearing instrument was typically 8-12 hours per day. The main outcome measure was the Four Alternative Auditory Feature speech recognition test. Acclimatisation was defined as an improvement over time in recognition score in the aided ear relative to the unaided ear. In the first experiment, subjects were tested with speech presented at an overall level of 60 dB SPL at three-week intervals over a post-fitting period of 24 weeks. The results revealed an improvement in performance in both ears over time but there was no consistent evidence of acclimatisation. In the second experiment, new subjects were tested with speech presented at an overall level of 60 dB SPL at four-week intervals over a post-fitting period of 24 weeks. In an attempt to reduce the practice effect observed in the first experiment, subjects were not given trial-by-trial feedback about their performance. On this occasion, there was a non-significant trend of improving performance in the fitted ear with no improvement in the control ear. It was hypothesised that the amplified level of speech in these experiments was not sufficiently different from that experienced in everyday life, prior to aiding, to show perceptual learning. In the final experiment speech was presented at overall levels of 55, 62 and 69 dB SPL and new subjects were tested at six-week intervals over a post-fitting period of 12 weeks. There was a significant improvement in performance over time with a trend towards greater acclimatisation at the highest presentation level.