Coloured filters, in the form of plastic overlays or tinted spectacle lenses, are in widespread use to alleviate visual stress (also known as Irlen Syndrome), a disorder posited to be a major cause of reading disability (Irlen, 1991; Wilkins, 2003). However, a recent systematic review (Albon et al., 2008) concluded that the evidence for the efficacy of coloured filters is insufficient to recommend the treatment. The existence of visual stress as a diagnostic entity has also been questioned (Royal College of Opthalmologists, 2009). This thesis first describes the various theoretical perspectives behind the use of coloured filters, and provides an in-depth review of the current evidence. A combined crossover study and randomised controlled trial of the coloured filters used by the Irlen Institute, the major proponent of the treatment, is then described. This experiment, which set out to avoid the methodological problems observed in previous trials - most importantly, double-blinding was employed - failed to find any evidence of visual stress, or for the statistically or clinically significant benefit of coloured overlays for reading rate or comprehension on two separate reading tests, in a sample of 61 Primary School-age children with reading problems. This was despite 77% of the sample having been diagnosed with visual stress by an Irlen diagnostician and prescribed coloured overlays. Visual stress theory and the current prevalence of the coloured filter treatment are discussed in the light of this new evidence.