This article reviews current knowledge regarding orthokeratology, also known as corneal refractive therapy. Modern orthokeratology using reverse-geometry gas-permeable lenses is an effective procedure for the temporary reduction of low to moderate myopia. The use of an overnight lens-wearing protocol provides an alternative to refractive surgery for many patients. Onset of the refractive effect is rapid, with observable changes within minutes and stability of effect after 7 to 10 days of treatment. The procedure appears to be fully reversible on cessation of lens wear. The orthokeratology effect is achieved through central corneal epithelial thinning and mid peripheral stromal thickening, although the cellular basis for these changes requires further research. Because of recent reports of severe corneal infections with overnight orthokeratology, the safety of the procedure is under active investigation, and it is clear that minimal clinical standards must be promulgated internationally to ensure a future for this approach to refractive correction.