The purpose of this study was to examine the relationships among middle school teachers' beliefs about collaboration, their rationale for using common formative assessments, and teacher characteristics that can explain these relationships. Seventy-six middle school teachers from two middle schools participated in the study. Findings indicate that teachers believe collaboration benefits instruction and assessment informs instruction. The findings suggest that age might play a role in the relationship between teacher beliefs and assessment, that the degree to which teachers get along with each other influences the success of a collaborative group, and that collaboration is not limited to structured meetings.