Focusing on the reflective exchanges of four test writers at an intensive language school within a public university, this study explored the effectiveness of a "lesson study" model on their professional development as test writers. Lesson study is a powerful tool for teachers' professional development and a modified version of it--a quiz study--was used in the process of developing, administering, reviewing the results and evaluating feedback of the test. Participants were audio recorded throughout the four stages of the quiz study cycle as they discussed a variety of topics. Moreover, content analysis of this data revealed that the test writers had negotiated two themes: assessment literacy and content-specific knowledge and skills. The significance of this study lies in understanding the nature of the quiz study process as well as test writers' professional development areas, which may provide valuable insight for all test writers, teachers, and teacher educators.