This paper presents the development of reliable measures of fidelity that represent critical elements of the curriculum and its use in assessing teachers' delivering of the treatment curriculum and identifying the existence of program differentiation between the treatment and control conditions. In order to address several issues that arise when assessing levels of implementation while confirming differentiation between treatment and control conditions through the use of well-developed measures, fidelity data collected from a preschool curriculum intervention have been analyzed to demonstrate an example of: (1) Creating psychometrically strong, comprehensive fidelity measures that represent the critical components of an intervention; and (2) Using those measures to view fidelity as something beyond a unitary construct by differentiating implementation of instructional constructs and activities across conditions. The analyses presented in this paper provide evidence that psychometrically valid set of measures may indeed be used to account for the multi-dimensional nature of implementation fidelity. Given the results of the internal consistency reliability testing, it is possible to conclude that the creation of accurate and valid models and associated fidelity indicators is complex and challenging. Although this model does represent key components of the curriculum, it has some limitations which will be discussed during the conference presentation. (Contains 1 figure and 1 table.)