This review identified only four studies comparing the use of one hearing aid with two. The studies were small and included participants of widely varying ages. There was also considerable variation in the types and degree of sensorineural hearing loss that the participants were experiencing.For the most part, the types of hearing aid evaluated would now be regarded, in high-income settings, as 'old technology', with only one study looking at 'modern' digital aids. However, the relevance of this is uncertain, as this review did not evaluate the differences in outcomes between the different types of technology.We were unable to pool data from the four studies and the very low quality of the evidence leads us to conclude that we do not know if people with hearing loss have a preference for one aid or two. Similarly, we do not know if hearing-specific health-related quality of life, or any of our other outcomes, are better with bilateral or unilateral aids.