The practice of classroom assessment occurs at the intersection of three teaching functions: instruction, classroom management, and assessment. Theory relevant to studying classroom assessment comes from several different areas: the study of individual differences (e.g., educational psychology, theories of learning and motivation), the study of groups (e.g., social learning theory, sociology), and the study of measurement (e.g., validity and reliability theory, formative and summative assessment theory). This article explores how intersections of these areas have played out in the classroom assessment literature over the last 20 years. Some literature has emphasized one practical function or theoretical tradition; some literature has blended several. Overlapping theoretical traditions are opportunities both for richness of understanding and for tensions and conflicts.