This exploration of the essential knowledge offered by research on teacher planning is divided into four parts. The first part deals with the kinds of knowledge derived from research on teaching. It is noted that teacher effectiveness researchers have been primarily concerned with producing knowledge about the observable behavior of effective teachers, while researchers on teacher thinking have pursued knowledge about how teacher's minds work as they plan, make decisions, teach, and reflect on experiences. A brief summary of the early history of research on teacher thinking is offered in the second part. Part 3, in which the results of selected studies of teacher planning are summarized, provides answers to three major questions: (1) What are the types and functions of teacher planning? (2) What models have been used to describe the process of planning? and (3) What is the relationship between teacher planning and subsequent action in the classroom? In the final part, conclusions derived from this research are discussed and recommendations are made concerning essential knowledge for beginning educators. (JD)