Many language teachers around the world design materials to supplement those they are provided with to address the needs of the learners in their particular context. This is a task which requires practical knowledge of various kinds relating to learners and language learning, teaching and materials design. However, while there is a growing body of research into the practical knowledge of language teachers, little of this is longitudinal and there is a lack of research into how teachers develop as materials designers. This article focuses on one teacher's growth as a DIY (do-it-yourself) designer of English language teaching materials during an in-service BA TESOL (teaching English to speakers of other languages) programme in the Middle East. Using qualitative case study methodology, I follow the teacher's development over three years, exploring changes in ideas and teaching practices. Implications for in-service language teacher education are discussed. URN: http://nbn-resolving.de/urn:nbn:de:0114-fqs1101334