This paper reports on the creation of a USMC Instructor Mastery Model and its utilization for setting performance standards and assessing instructor performance. The Mastery Model is derived from the Dreyfus and Dreyfus model of cognitive skill acquisition. It makes the path to mastery explicit by specifying how individuals progressively develop into high performers and what indicators can be observed and assessed during each of five stages of development. To customize the model for Marine instructors, reviews of the literature and other services' instructor development approaches were conducted. Semi-structured interviews were conducted with 93 highly skilled instructors at 15 USMC learning institutions. Thematic analysis and card sorts were employed to understand what a skilled Marine instructor does and define 10 Key Performance Areas (KPAs). A second analysis examined how an instructor performs at the five stages of development and yielded performance indicators for each KPA at each stage. Outcomes were examined against teacher competencies from the literature to compare military and civilian instruction. The final set of KPAs has been adopted by TECOM and included in the Train the Trainer Training and Readiness (T&R) Manual in the form of five T&R events and five appended learning outcomes. The next step is to produce an Instructor Assessment Battery and observation rubric to enable assessment of individuals' progressive skill development as well as program- and policy-level impact assessments.