This study explored the development and processes of thinking, planning, and decision making of two preservice teachers as they progressed through their teacher education program. The effects of training in the systems approach on two preservice teachers' thinking about instruction and on their actual instructional planning documents were examined. Also investigated was the relationship between these teachers' training in instructional systems design and the approaches to teaching and planning for instruction presented in the pedagogical courses of their teacher education program. Two preservice teachers enrolled in an instructional design course participated in this study. Data were collected over a 3-year period; the primary data sources were interviews, students' notes, journals, assignments, projects and lesson plans, an instructional design survey, observations of student teaching, and personal communication with the students' university supervisors. While the students' beliefs did not change drastically over the course of their teacher education program, they refined, expanded, and validated their beliefs by gaining theoretical and practical knowledge. The results of the instructional design survey indicated that both students used instructional design principles, and to some extent procedures, thereby reflecting a systems perspective on instruction. Analysis of the students' lesson plans and their execution showed that the lesson format taught by the university, the cooperating teacher's style, and the textbook materials highly influenced the way both participants designed their lessons. The lesson plans and the statements that these preservice teachers made revealed an increasingly complex but also integrated conception of the teaching-learning process. Six tables present study data. (Contains 34 references.) (AEF)