This study examines the psychometric properties, reliability, and validity of a measure designed to assess fidelity of implementation of the "Responsive Classroom"[R] ("RC") approach. The Classroom Practices Teacher Survey (CPTS) assesses teachers' use of the "RC" approach, a social and emotional learning (SEL) intervention currently under investigation in an efficacy study. The present study addresses three research goals concerning the CPTS: (1) understanding psychometric properties; (2) examining reliability; and (3) building evidence for validity. This study draws upon two measurement frameworks: Classical Test Theory (CTT) and Item Response Theory (IRT). CPTS item properties were based on a multidimensional Partial Credit Model (PCM) comprised of three dimensions representing important aspects of the "RC" approach (Morning Meeting, Rules and Logical Consequences, and General Classroom Practices). Additionally, Discriminant Function Analysis provided evidence that CPTS dimension scores derived from the PCM discriminated between non-"RC" and "RC" teachers with eighty-eight percent accuracy. The present findings support the CPTS as a valid measure of "RC" fidelity of implementation. The discussion places the findings in context of other research examining fidelity of implementation and in relation to other studies of SEL interventions. As a reliable and valid measure of FI, the CPTS will serve to strengthen causal inferences in the context of the "Responsive Classroom" Efficacy Study. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]