In order to compare the differences in cognitions of experienced and beginning teachers, the perceptions of six experienced teachers, six student teachers, and six probationer teachers (in their first year of teaching after college training) were studied at various times during the course of a school year, using a variety of methods (interview, repertory grid techniques, and stimulated recall commentaries on parts of lessons involving interaction with individual pupils or small groups). The findings from this exploratory work suggest that teachers possess qualitatively different types of information about pupils. Some types of information appear to be more useful to teachers in some contexts than others, although different types of information are often combined to guide their classroom actions. Teachers' perceptions can be classified into four major categories: (1) knowledge about pupils in general; (2) general knowledge about particular pupils; (3) specific knowledge about pupils; and (4) knowledge related to diagnostic/remediation routines. When comparing experienced teachers' knowledge with that of student teachers and probationer teachers, it seems that beginning teachers start out with very little of any of these types of knowledge. The type of knowledge that student and probationer teachers acquired most rapidly was general knowledge about particular children. (JM)