The present paper gives a review of diary methods in clinical psychology and psychotherapy. After a brief discussion of the history and the development of the diary method a description of the methodology and technology of diaries in their scientific and practical uses is given. The main part of the paper presents an overview of instruments used in research and practice in clinical psychology concerning mood and affectivity, anxiety disorders, pain, psycho-physiological states, eating disorders and alcoholism. Additionally, methodological advantages and problems of these tools are discussed. Finally strategies for the assurance and enhancement of the methodological quality of diary data are discussed. The paper comes to several conclusions: (1) although many instruments have been developed during recent years for research and practical aims, they are concerned with only a few of the many clinical phenomena; (2) although some diaries achieve favourable psychometric results, the reliability and validity of most of the instruments has not yet been explored in a satisfactory manner; (3) although most diaries would also be useful for practical reasons, many deal with research aims. Therefore, future work should focus on practical use as well as on psychometric testing. Copyright © 2002 John Wiley & Sons, Ltd.