The Chicago Public Schools have been developing and implementing a citywide instructional program of continuous progress/mastery learning in reading. It includes the delineation of a K-8 continuum of instructional objectives; a criterion-referenced testing program which serves as the basis for instruction, promotion, and administrative monitoring; and a mastery learning instructional system: the Chicago Mastery Learning Reading materials for teachers. The Chicago mastery learning model differs from other mastery learning models in three major regards: (1) it welds mastery learning theory with theories of cognition and learning and with concepts relating to instruction; (2) it emphasizes group instruction; and (3) its delivery system emphasizes materials development rather than staff development. In evaluating the effects of this innovation, the theory of mastery learning was used to infer specific changes and patterns of changes which ought to occur in achievement test scores, thus enriching the interpretation of that data. (Author/BW)