This report reviews the scholarly and research evidence supporting the construct labeled modeling and explaining content (MEC), which is measured via a performance assessment in the "ETS"® National Observational Teaching Examination (NOTE) assessment series. This construct involves practices at the heart of teaching that deal with how teachers model and provide explanations for knowledge and skill in the course of instruction. The construct, supporting evidence, and measurement are organized around 4 dimensions: framing the work; demonstrating targeted processes, strategies, or techniques; supplying explanations and other narration during demonstrations; and using language, terminology, and representations. The paper concludes with a description of the MEC performance assessment. The following is appended: Sample Released Task for Modeling and Explaining Content (MEC). [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 "Leading a Classroom Discussion: Definition, Supportive Evidence, and Measurement of the 'ETS'® National Observational Teaching Examination (NOTE) Assessment Series. Research Memorandum No. RM-16-09" (ED570574).]