This article develops a validity argument approach for use on observation protocols currently used to assess teacher quality for high-stakes personnel and professional development decisions. After defining the teaching quality domain, we articulate an interpretive argument for observation protocols. To illustrate the types of evidence that might compose a validity argument, we draw on data from a validity study of the Classroom Assessment Scoring System for secondary classrooms. Based on data from 82 Algebra classrooms, we illustrate how data from observation scores, value-added models, generalizability studies, and measures of teacher knowledge, student achievement, and teacher and student beliefs could be used to build a validity argument for observation protocols. Strengths and limitations of the validity argument approach as well as the issues the approach raises for observation protocol validity research are considered. (Contains 9 tables and 3 figures.)