This study provides evidence of the positive impact of library media centers (LMCs) on academic achievement in 221 Colorado public schools during the 1988-89 school year. The importance of the library media specialist's instructional role is also verified. In contrast to previous research on the relationship, this study uses schools rather than students as units of analysis, considers service outputs as well as resource inputs, and rules out the effects of selected school and community conditions which might have explained away this relationship. Highlights of the findings include: (1) where LMCs are better funded, academic achievement is higher, whether their schools and communities are rich or poor and whether adults in the community are well or poorly educated; (2) better funding for LMCs fosters academic achievement by providing students access to more library media staff and larger and more varied collections; (3) among predictors of academic achievement, the size of the LMC staff and collection is second only to the absence of at risk conditions, particularly poverty and low educational attainment among adults; (4) library media expenditures and staffing tend to rise and fall with total school expenditures and staffing; (5) students whose library media specialists participate in the instructional process are higher academic achievers. Also included in the report are a current and comprehensive annotated bibliography on this subject and a timeline chronicling the evolution of such studies. (Author/KRN)