This study reports the development of an instrument intended to measure mathematics teachers' knowledge of Cognitively Guided Instruction (CGI). CGI is a mathematics professional development framework based on how students think about and solve problems and how that knowledge guides instruction for developing mathematical understanding. The purpose of this study was to (a) analyze and revise the original CGI Teacher Knowledge Assessment (CGI TKA), (b) administer the revised CGI TKA, and (c) analyze the results from the revised CGI TKA. As part of the revision of the original CGI TKA, distractor analysis identified distractors that could be improved. Experts in CGI content were interviewed to identify ways in which the content of the CGI TKA could be improved, and some new items were created based on their feedback. Formatting changes were also made to administer the assessment electronically.After the original CGI TKA was revised, the revised CGI TKA was administered to teachers who had been trained in CGI. Two hundred thirteen examinees completed the revised CGI TKA and the results were analyzed. Exploratory and confirmatory factor analyses showed 21 of the items loaded adequately onto one factor, considered to be overall knowledge of CGI. The Rasch model was used to estimate item difficulty and person abilities as well as to compare models using dichotomous and partial credit scoring. Advantages and disadvantages of using partial credit scoring as compared to dichotomous scoring are discussed. Except under special circumstances, the dichotomous scoring produced better fitting models and more reliable scores than the partial credit scoring. The reliability of the scores was estimated using Raykov's rho coefficient. Overall, the revised CGI TKA appears to validly and reliably measure teachers' CGI knowledge.