Construct: We sought to evaluate the quality of Team-Based Learning facilitation in both large and small group settings. Background: Team Based Learning (TBL) is an increasingly popular small group instructional strategy in health science education. TBL facilitation skills are unique and differ from those needed to lecture or facilitate other types of small groups. Measuring facilitation skills and providing feedback to TBL instructors is important, yet to date no valid instrument has been developed and published for this purpose. Approach: We created an 11-item instrument (ratings of each item on a 7-point scale) designed to assess TBL facilitation skills, considering major sources of validity. Twelve experts in TBL facilitation and training developed the content of the FIT. To ensure response processes were valid, we used an immediate retrospective probing technique with 4th year medical students who were not part of the study. The Facilitator Instrument for Team-Based Learning (FIT) was piloted with 2,840 medical students in 7 schools in large (year 1 and 2) and small (year 3) courses. The internal structure of the FIT was analyzed. Results: In total, 1,559 and 1,281 medical students in large and small TBL classes, respectively (response rate 88%) rated 33 TBL facilitators. The composite mean score for the FIT was 6.19 (SD = 1.10). Exploratory factor analysis and Cronbach's alpha indicated that all items loaded on 1 factor, accounting for 77% of the item variance. Cronbach's alpha for the 11 items was 0.97. Analysis of facilitator variables and course context indicated that FIT scores were statistically significantly correlated with type of class (pre-clinical or clinical) and size of class as well as the facilitator enjoyment in using TBL as a method. Gender and the amount that facilitators used TBL each year was weakly correlated, with other factors not correlated (years facilitating TBL, confidence in facilitating TBL, and age). Conclusions: Analysis of FIT scores from 2,840 medical students across multiple institutions and teaching settings suggests the utility of the FIT in determining the quality of TBL facilitation across a range of medical education settings. Future research is needed to further analyze course contexts and facilitator variables that may influence FIT scores with additional facilitators. Additionally, FIT scores should be correlated with additional measures of TBL facilitator quality, such as direct observations, especially if these data are used for summative decision-making purposes.