This paper provides a description and rationale for a performance assessment of a teaching practice--leading a classroom discussion (LCD)--included in the ETS® National Observational Teaching Examination (NOTE) assessment series. In this assessment, candidates interact with a small class of virtual students represented by avatars in a computer-based, simulated classroom. The five avatars are enacted by a single simulation specialist who has been trained and certified on the particular task presented, either in elementary English language arts or mathematics. The paper defines and describes the construct of LCD, then provides a review of the research and scholarly literature that supports the importance of this practice for effective teaching, and finally describes how the construct is measured in the NOTE assessment. The following is appended: Leading Classroom Discussion (LCD) Sample Task. [Some of the content in this report is used in the following companion reports: "Eliciting Student Thinking: Definition, Research Support, and Measurement of the 'ETS'® National Observational Teaching Examination (NOTE) Assessment Series. Research Memorandum No. RM-16-06" (ED570608); and "Modeling and Explaining Content: Definition, Research Support, and Measurement of the 'ETS'® National Observational Teaching Examination (NOTE) Assessment Series. Research Memorandum No. RM-16-07" (ED570617).]