By focusing on letter-writing the article provides an example of how a technology of the word could be used as a panoptic technology of observation and reformation. The article discusses the practice of using letters in monitoring children's development in early twentieth-century reform school education. It is based on the author's study of Finnish reform school education and draws on a collection of children's letters and educational writings in the archives of the Vuorela State Reform School, which was established for delinquent girls in 1893. The article focuses on one of the most effective devices for using letters in educational purposes that was introduced in reform school education: the notebook on correspondence between girls and their families. The notes were compiled by the personnel, who carefully examined all in-coming and outgoing mail and who also supervised inmates' letter-writing. How was letter-writing connected to wider pedagogical aims and what kind of encounters between reformatory pupils and their teachers can be found in the surviving material? The article argues that the systematic examination of inmates' letters was not only about control, but more importantly about education and observation. Notes on inmates' letters include short commentaries concerning girls' behaviour and their character. As is shown in the article, the letters were archived and annotated in order to gain a deeper comprehension of the author's personality and her progress in the institution. The practice of monitoring letter-writing at Vuorela will be contextualised in the wider history of reform school education. In the case of Vuorela state reform school letter-writing appears as a social practice defined by the institutional context. The pedagogical aims of reforming the pupils produced a specific reformatory culture of routines, rituals and restrictions. Letters had a specific role in the reformatory culture due to the all-embracing aim of surveillance and cure. Letters and education in letter-writing were taken on as a technique of not only observing but also reforming. (Contains 43 footnotes.)