This project will investigate the relatively unexplored area of how beliefs, knowledge, decisions and actions of people in the process of becoming middle school teachers of mathematics change during their training and first year of teaching as they begin to make the transition from novices to experienced teachers. In conceiving the study, these investigators bring together the concepts of (1) teacher beliefs and knowledge, (2) decisions and instruction strategies used by teachers, and (3) environmental influences including both contextual and social factors present in the schools where teachers study and work. These are used to form a framework for understanding the changes over time in the manner in which new teachers identify and respond to situations in which problems and opportunities arise during instruction. The study will employ a case study approach to describe the detail and complexity of the succession of decisions and actions of novice teachers during the final year of preservice preparation and first year of teaching middle school mathematics. Particular attention will be given to the ways these teachers plan, implement, and reflect upon the mathematics instructional units they teach.