Extant coverage of multi-tiered systems of support (MTSS) in the research literature has tended to focus on student outcomes as opposed to the process of adult implementation. This conceptual article presents a model of process evaluation as a means to assess the fidelity of implementation of a multi-tiered system of supports (MTSS) framework to support instructional practices in reading in schools. The article begins by defining MTSS and its core components and overviewing the applicability of process evaluation in assessing MTSS implementation "and" outcomes. Novel process evaluation tools are presented, including examples of their application in evaluating MTSS implementation for tiered reading instruction. Implications are drawn regarding how MTSS process evaluations can be conducted by school psychologists or other evaluators in school settings, and why it can be valuable to do so.