A descriptive case study was designed to investigate how classroom teachers plan and implement instructional adaptations and accommodations for mainstreamed students. Two elementary school classroom teachers, each receiving two mainstreamed students with mental retardation, were observed to determine factors and strategies involved in making instructional adaptations and accommodations. Preactive teaching processes, including consultative and collaborative planning activities, and classroom teacher perceptions about mainstreaming practices were examined. Results indicated five primary types of instructional adaptations: acquisitional, parallel, enabling-social, enabling-academic, and structural. In addition, two other strategies were observed: accommodations and return to the resource room for mainstreamed students. Findings indicated acquisitional adaptations facilitated the integration of mainstreamed students, structural adaptations powerfully affected classroom climate and students' interpersonal relationships for both mainstreamed and nondisabled students, and parallel adaptations had both positive and negative social and instructional implications for mainstreamed students. Enabling-social and enabling-academic adaptations produced increased mainstreamed student participation in classroom instructional activities and experiences. Benefits were found for nondisabled students as well. Preactive teaching processes included variable use of consultative and collaborative strategies and some use of instructional adaptation planning’ routines. Teachers'! planning processes changed over the course of the year; adaptation-making processes moved from the preactive to the interactive teaching phase. Teachers' reported perceptions of mainstreaming varied during the study, but generally remained positive. Based on these results, a taxonomy of descriptions of instructional adaptations as well as descriptions of classroom teachers' planning processes as strategies for mainstreaming students with mental retardation in general education classroom instructional activities and classroom teachers' perceptions of the mainstreaming program were derived.Ed. D.