Consultation researchers have long recognized the importance of assessing fidelity of intervention implementation, including the fidelity with which both consultation procedures and behavioral intervention plans are delivered. However, despite decades of discussion about the importance of assessing for fidelity of implementation in intervention delivery, the empirical foundation lags far behind in systematic efforts to incorporate reliable, valid, and conceptually meaningful fidelity measurement into its procedures. The methods used to capture elements of implementation are often incomplete, imprecise, and of questionable reliability. Among the methods commonly used to assess intervention fidelity in consultation(self-report, permanent products, direct observation), there exists little to no research documenting their psychometric adequacy. This article explores issues surrounding the assessment of fidelity in consultation research, including its rationale and role in consultation and intervention science. Methods for conceptualizing and assessing fidelity, psychometric issues, and research needs are identified. The results of a descriptive, exploratory study tapping the reliability of fidelity assessment measures within the context of a large-scale efficacy trial are presented, with a call for rigorous research to advance the consultation field. (Contains 2 tables and 1 footnote.)