A discussion of diary studies, research on second language learning and teaching that uses student journals about the learning experience, defines the method, reviews a number of such studies, and examines the pros and cons of this approach. It is noted that diary studies are first-person case study research, a form of empirical (data-based) research enhanced by introspection and analysis. Introspection can be concurrent with the learning event, immediate retrospection, or delayed retrospection. Diary studies span this continuum. Problems associated with the method include: sampling limitations (small samples, biased subjects); data collection issues (those inherent in self-reporting, bias in retrospection, limitations of introspection, quality and breadth of entries, and commitment of the diarist); and data analysis concerns (limited generalizability of small quantities of information, inappropriateness of causal statements, and all problems typically associated with qualitative analysis). The method is seen to have advantages for: teachers, in re-examining the language learning process; learners, for whom diaries are a safety valve for frustration, promote awareness of learning processes and pitfalls, and are evidence of progress; and language learning research, by adding to the knowledge base and also revealing new issues and factors to be considered.