Previous research has shown that questioning is a key strategy that facilitators use to promote discussion in Problem-Based Learning (PBL). Yet, there is a lack of detailed understanding on what questions facilitators ask and how those questions affect discussion. In this study we examined different types of questions that experienced facilitators asked to promote discussion of teaching problems in professional development for science teachers. We videotaped six PBL sessions facilitated by three pairs of experienced facilitators. Data analysis showed that facilitators asked a set of questions to initiate and advance PBL discourse, including questions to solicit ideas, to reframe ideas, to clarify ideas, to push for elaboration, to check for interpretation, and to connect to teachers' classroom practice. This study has implications for the development of PBL facilitators.