This article aims to investigate teachers' perception of sources of knowledge in their continuing professional development (CPD). It investigates whether the perceived importance of a source can be related to its trustworthiness. Knowledge sources comprise institutions and colleagues, who produce knowledge for teachers' development. The issues are examined by analysis of questionnaire studies on teachers' CPD with a sample size of 711 teachers in Germany and Sweden. The results show that a knowledge source's trustworthiness is a relevant and significant predictor for its importance in teachers' CPD. However, the national context in which teachers work also has a considerable influence. (Contains 8 tables.)