There has been an increased focus on the tenets of implementation science in special education research, namely on the degree to which interventions are implemented as intended (i.e., fidelity of implementation) and factors that influence fidelity of implementation. For complex interventions, such as the "Self-Determined Learning Model of Instruction" (SDLMI), there is an ongoing need to refine conceptualizations of fidelity of implementation the factors that influence it in inclusive educational contexts. Therefore, the purpose of this paper is two-fold: (a) to describe a framework for conceptualizing fidelity of implementation factors that influence fidelity when evaluating a complex intervention like the SDLMI and (b) to present initial content validity and internal consistency of a new measure designed to assess specific aspects of fidelity of implementation of the SDLMI in inclusive, general education classrooms. Implications and future directions for research in implementation science, special education, and self-determination interventions are described. [This paper will be published in "Inclusion."]