3-11% of children and adolescents suffer from a reading andor spelling disorder. Their poor written-language skills markedly impair their scholastic performance and are often associated with other mental disorders. A great deal of uncertainty still surrounds the question of the appropriate methods of diagnosis and treatment. We systematically searched for pertinent publications in databases and literature reference lists, summarized the evidence in six tables, and examined some of it in a meta-analysis. Recommendations were developed in a consensus conference. A reading and/or spelling disorder should only be diagnosed if performance in these areas is below average. It should be determined whether an attention deficit-hyperactivity disorder, anxiety disorder, or disorder of arithmetical skills is also present. Reading and spelling performance should be reinforced with systematic instruction about letter-sound and sound-letter correspondences, letter-syllable-morpheme synthesis, and sound-syllablemorpheme analysis (g' = 0.32) (recommendation grade A). Spelling ability responds best to spelling-rule training (recommendation grade A). Irlen lenses, visual and/or auditory perceptual training, hemispheric stimulation, piracetam, and prism spectacles should not be used (recommendation grade A). Evidence- and consensus-based guidelines for the diagnosis and treatment of reading and/or spelling disorders in children and adolescents are now available for the first time. Reading and spelling abilities should be systematically and comprehensively reinforced, and potential comorbid disorders should be sought and treated appropriately. The efficacy of many treatments now in use has not been documented; if they are to be used in the future, they must be tested in randomized, controlled trials. For adult sufferers, adequate diagnostic instruments and therapeutic methods are not yet available.