The purpose of this study was to compare the sources of knowledge that expert golf instructors and expert tennis instructors considered most useful for teaching. Twenty-three expert golf instructors and twenty expert tennis instructors participated in two separate studies. Both studies were part of a larger examination of expertise in sport instruction. The knowledge sources components of both studies used a Q-Sort data collection technique. This method of data collection allows for the comparison of usefulness and importance of the source of knowledge across the two areas of sport instruction. Each instructor (golf and tennis) was asked to rank order either eleven (golf) or thirteen (tennis) cards, with each card denoting a possible source of knowledge. After sorting the cards the instructors were interviewed to determine the rationale for their particular card ranking. It must be noted at anytime during this stage of data collection the participant could modify and/or rearrange the order of the cards. Analysis of the average rank scores revealed Grand Rank orders and resulted in the formation of three main categories of importance: primary, secondary, and tertiary. Results of ranked data analysis revealed that although the order was not exactly alike, the three most important and useful sources of knowledge for both tennis and golf instructors were identical. Further statistical analysis using Chi-squared and Spearman correlation methods indicated that there were no significant differences between the groups of instructors, and that there was a high correlation (0.85) in the rankings of knowledge sources between the two groups. It is interesting to note that the top three knowledge sources for both groups are all related to social interaction. Interview data corroborate the statistical findings in the instructor's own words. Therefore the main findings of this study were that the sources of knowledge that both expert golf and tennis instruction deemed must useful to develop domain specific knowledge were the same. In addition the study shows that expert instructors in both goal and tennis have drawn their expertise form the same primary sources of knowledge.