While it is generally recognized that teacher research can be a very beneficial form of continuing professional development (CPD), there is still relatively limited research available on the impact this activity has on teachers' self-efficacy beliefs, which are of interest to educational psychologists because, while being open to constant change, these beliefs influence the way in which knowledge is transformed into action. There is also a relative lack of available research into how teachers develop as researchers; the processes whereby they gain practical knowledge and more positive self-efficacy beliefs in planning, conducting, analysing, presenting and writing up research require further exploration. This qualitative multi-case study addresses these issues, exploring the development of three in-service teachers of English on a foundation programme at a Turkish university. Findings reveal that engaging in CPD which directly benefited their learners helped all three teachers develop positive teachers' self-efficacy beliefs and deeper practical knowledge in relation to the specific tasks that concerned them. Furthermore, from a starting point of having low self-efficacy beliefs in conducting practical research, which reflected their lack of prior knowledge in this area, they all became more efficacious as they gained research experience and developed practical knowledge of research. This study highlights the benefits, then, of helping teachers become more efficacious through CPD that engages them as knowledge-generators. Enthusiastic mentoring, autonomy support and the opportunity to present their research more widely all helped the teachers in this Turkish context to develop.