Complex Instruction (CI) is a set of principles and practices for designing and facilitating equitable groupwork. Originally developed to advance racial equity in United States primary schools, CI is now used to support students of many ages across many disciplines. We report on a systematic review of CI-focused research in the U.S. up to the year 2021, with a focus on which major components of the CI pedagogical approach (i.e. groupworthy tasks, multiple ability treatments, assignments of competence, group roles, group norms) remain widely recognised and which may have been adapted, attenuated, or abandoned during its implementation. Out of 117 peer-reviewed articles from the U.S., our findings show few research articles attend to all five major components of CI. The one component most prevalent was a groupworthy/complex task (n = 91), the least mentioned was multiple abilities treatment (n = 36), and only 12 articles discussed every major component of CI. The least discussed components of CI were those that attended to status interventions -- the issue that initially motivated the development of CI. While the literature indicates the effective implementation of several aspects of CI, the review also suggests implications for future research and teacher education that attend to CI's theoretical foundations.