The use of personal documents in social science research, and in particular, methods and limitations of journal keeping in research on teaching planning, are discussed. Field studies are reported in which teachers were asked to record their planning deliberations and the accompanying thoughts in a personal journal. While it became apparent that for many of the teachers journal writing was a valuable tool in their planning and teaching, the question arose as to whether or not journal writing, as a personal document, can be defended as a legitimate inquiry mode for studying human experience, and if so, what safeguards must be instituted to defend the quality and validity of the research. A review of social science research methodology studies revealed differing opinions on the validity of using personal documents as a basis for drawing firm conclusions on the thinking and planning processes. It is concluded that while the journal is an imperfect instrument for learning about human thought, journal keeping as a research tool is a benign, generative, and economical device for recording teachers' descriptions of and insights about their planning and teaching. (JD)