We summarize some of the most important findings from research evaluating the hypothesized causes of specific reading disability (ÔdyslexiaÕ) over the past four decades. After outlining components of reading ability, we discuss manifest causes of reading difficulties, in terms of deficiencies in component reading skills that might lead to such difficulties. The evidence suggests that inadequate facility in word identification due, in most cases, to more basic deficits in alphabetic coding is the basic cause of difficulties in learning to read. We next discuss hypothesized deficiencies in readingrelated cognitive abilities as underlying causes of deficiencies in component reading skills. The evidence in these areas suggests that, in most cases, phonological skills deficiencies associated with phonological coding deficits are the probable causes of the disorder rather than visual, semantic, or syntactic deficits, although reading difficulties in some children may be associated with general language deficits. Hypothesized deficits in general learning abilities (e.g., attention, association learning, cross-modal transfer etc.) and low-level sensory deficits have weak validity as causal factors in specific reading disability. These inferences are, by and large, supported by research evaluating the biological foundations of dyslexia. Finally, evidence is presented in support of the idea that many poor readers are impaired because of inadequate instruction or other experiential factors. This does not mean that biological factors are not relevant, because the brain and environment interact to produce the neural networks that support reading acquisition. We conclude with a discussion of the clinical implications of the research findings, focusing on the need for enhanced instruction.

Reading impaired first graders were given daily tutoring as a "first cut" diagnostic to aid in distinguishing between reading difficulties caused by basic cognitive deficits and those caused by experiential deficits. Reading achievement in most of these children was found to be within or above the average range after one semester of remediation. Children who were difficult to remediate performed below both children who were readily remediated and normal readers on kindergarten and first-grade tests evaluating phonological skills, but not on tests evaluating visual, semantic and syntactic skills. The results are consistent with convergent findings from previous research suggesting that reading problems in some poor readers may be caused primarily by phonological deficits. Specific reading disability is conventionally defined as severe difficulty in learning to identify printed letters and words in children who have at least average intelligence and who are not impaired by general learning difficulties

Children at risk for early reading difficulties were identified on entry into kindergarten, and half of these children received small-group intervention two to three times a week during their kindergarten year. The other half received whatever remedial assistance was offered by their home schools. These children were again assessed at the beginning of first grade, and those who continued to have difficulties in reading received either one-to-one daily tutoring offered by project teachers from the beginning to the end of first grade or whatever remedial assistance was offered by their home schools over the same time period. All target children were periodically assessed through the end of third grade. Results suggest that either kindergarten intervention alone or kindergarten intervention combined with first-grade intervention are both useful vehicles for preventing early and long-term reading difficulties in most at-risk children.

Entry-level kindergartners in classrooms from five middle class school districts were given a test of letter identification and children who scored at or below the 30th percentile on the test were classified as ''at risk'' for early reading difficulties. Half of these children were randomly assigned to a project-based intervention condition where they received supplementary intervention in small groups until the end of their kindergarten year. The other half received whatever remedial services were available at their home schools and literacy skills development in both groups was tracked throughout kindergarten. All available at-risk children were again assessed at the beginning of first grade and dichotomized into a ''continued-risk'' group and a ''no-longer-at-risk'' group using a composite measure of basic word level skills. Normal reader controls were also identified using the same measure. Children in the continued-risk group received either project-based intervention (one-to-one tutoring 30 min daily) or school-based intervention throughout first grade. Intervention for project treatment children was discontinued at the end of first grade and literacy development in all groups was tracked until the end of third grade. The present study focused on literacy development in children who received only project-based kindergarten intervention or both (project-based) kindergarten and first grade intervention, relative to the normal reader controls. Of special interest was the question of whether measures of response to intervention would more effectively distinguish between continued-risk and no-longer-at-risk children than would kindergarten screening measures, measures of intelligence, or measures of reading-related cognitive abilities. Results indicated that the RTI