Fidelity of implementation (FOI) is the extent to which delivery of an intervention adheres to the original intent of the program designer. FOI in educational studies is hindered by the lack of a universally agreed upon definition or set of criteria for measurement. With the increasing need for justification of reliability and validity of curricular interventions, steps to define, conceptualize, measure, assess, and report fidelity are crucial. Few studies identify or even disclose fidelity measures, much less the critical components of intervention; therefore, it is difficult to make comparisons between studies. The field of educational research, however, is awakening to the importance of the study of implementation and requirements for assessment of fidelity. The purposes of this study were to determine: (a) the extent to which observed lessons were implemented in a curricular intervention designed for third-grade gifted students, (b) how observed FOI related to self-reported FOI, and (c) why teachers made adaptations to the curricular intervention. Thirty teachers implementing one of the lessons in the curricular intervention were observed and interviewed. Results from the observations and interviews indicated that teachers were able to implement the intervention with a high moderate degree of fidelity. Furthermore, identification of the critical components of the intervention found that overall, teachers made adaptations to big ideas, procedural tasks, and activities involving reading skills. This finding provided valuable information on program feasibility and for professional development for observers, raters, and implementers. Findings also indicated that self-reported FOI was highly correlated with observed FOI suggesting self-report may be a reliable alternative to costly direct observations when measures are clear and precise. Moreover, the level at which a teacher was able to self-report fidelity similar to the observer fell on a continuum from "very closely" to "not very closely" depending on their observed fidelity level. The higher the observed fidelity the more likely the teacher was to report in a manner consistent with the observer. Finally, the reasons given by teachers for making changes supported the on-going construction of an empirical base for FOI studies. Findings indicate that there are at least 18 contextual and three programmatic factors that influence teachers' decisions, and thus, FOI. The implementation process was affected by variables related to internal factors such as teachers, classrooms, and school capacity, in addition to external factors such as intervention design/complexity and educational policies from multiple levels. Overall, teachers made decisions for adaptations based on student ability or skill; however, whether the decisions were in accord with the program model often depended on whether or not the teachers backed the decision on actual performance and assessment, as opposed to perceived student ability or skill. High fidelity teachers made more decisions based on actual performance and assessment. Moderate fidelity teachers gave reasons similar to high and low fidelity teachers, but differed in that they often unintentionally omitted a component because they felt pressured by time constraints. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]