Although there has been increased interest in what constitutes effective professional development (PD) for in-service teachers in recent decades, the literature indicates that the issue continues to promote ongoing debate. Based upon the findings of previous research, this qualitative study set out to determine the extent to which, how, and why a PD course was considered effective in its contribution to the development and practice of the 28 in-service EFL teachers in Israel who participated in the course. Data from written reflective accounts, interviews, and field notes were collected and analysed. The findings identify various ways in which the course was considered effective, and reasons for such effectiveness, that, in turn, indicate the need for PD courses to be tailored to the current needs of practitioners as perceived by the course participants themselves.