This article presents the development and the technical and conceptual characteristics of two of the three measures used in the SET project, to discuss how they relate to each other, and to present evidence of their concurrent validity. The Pathognomonic-Interventionist (P-I) Interview yields rich descriptions of teachers' experiences with one or more students with special education needs included in their classes. The scoring system infers teachers' beliefs about disabilities, and the teachers' self-described instructional practices in working in inclusive elementary classrooms. The Classroom Observation Scale (COS) is a detailed observation by two third-party observers of teacher-student interactions during instruction in core subjects in the regular classroom when students with SEN are present. Based on criteria for effective instruction, the COS yields a quantitative score of teaching practices in four categories, as well as Predominant Teaching Style, a measure of the quality of instructional interactions with individual students during the lesson. In this article the relationships between the P-I and COS measures are explored, asking, for example, whether the COS validates teachers' self-reports about their inclusive practice, and whether the P-I scale reflects differences observed in teachers' practices. A research agenda to extend this inquiry is proposed. [For "The Supporting Effective Teaching Project: 1. Factors Influencing Student Success in Inclusive Elementary Classrooms," see EJ1200792.]