This conceptual paper explores fundamental issues in assessing implementation. Fundamental issues have received attention in the literature, but have yet to be compiled in a decision framework. Assessing implementation provides important information regarding program feasibility, interpretation of program outcomes, and program theory. Implementation assessment is of particular importance in practical or real-world settings, which lack the control of clinical trials or laboratory settings. Five fundamental issues identified in literature are presented in the form of decisions evaluators make when planning implementation assessment. Fundamental decisions include (1) a theoretical approach of fidelity or adaptation; (2) a focus on delivery or receipt; (3) measurement of quantity, quality, or structure; (4) consideration of implementation globally or of specific program components; and (5) systematic manipulation of implementation or naturalistic observation. Fundamental decisions are explored using actual or hypothetical examples in education and prevention programs. #ImplementationAssessment #2011Conference #FidelityofImplementation #ResearchonEvaluation #HowTo