Evidence has accumulated regarding the efficacy of coloured lenses in reducing symptoms in individuals susceptible to visual perceptual problems (visual stress) and yet there is still scepticism from some quarters. This article discusses the mechanisms which have been advanced to explain visual stress, including magnocellular deficits, binocular instability and cortical hyperexcitability. Each mechanism is discussed in some detail with references to the supporting scientific literature. The authors then discuss the use of coloured lenses in specific learning difficulties, giving examples of randomised controlled trials and highlighting where further research is required. The role of coloured lenses in migraine, autism and epilepsy is also covered.