Evaluation reports increasingly document the degree of program implementation, particularly the extent to which programs adhere to prescribed steps and procedures. Many reports are cursory, however, and few, if any, fully portray the long and winding path taken when developing evaluation instruments, particularly observation instruments. In this article, we describe the development of an observational method for evaluating the degree to which K-12 inquiry science programs are implemented, including the many steps and decisions that occurred during the development, and present evidence for the reliability and validity of the data that we collected with the instrument. The article introduces a method for measuring the adherence of inquiry science implementation and gives evaluators a full picture of what they might expect when developing observation instruments for assessing the degree of program implementation. (Contains 2 figures and 1 table.)