School counselors are uniquely trained and positioned within schools to use data for the purpose of addressing opportunity and achievement gaps present in many schools. Even with this position, the literature for school counseling remains focused on reasons to use data rather than how to use data within a school counseling program. This study examines the relationship between professional self-efficacy and levels of data use to understand an aspect of why school counselors might not use data to promote school counseling program goals and advocacy. Participants are 426 public school counselors from Missouri. Participants are mostly female (89.4%) and have 7-18 years of experience as school counselors (44.8%). Simple and multiple regression analysis is used to examine the relationship between self-efficacy and data use. Additional factors are self-efficacy in mathematical concepts and the involvement of participants in continued training on the use of data. Statistically significant variation in school counselor's data use is attributed to self-efficacy (R[superscript 2] = 0.247, p < 0.01). The addition of higher self-efficacy in math concepts (R[superscript 2]=0.370) and participation in professional development (R[superscript 2]=0.40) increases the strength of the model (p <0.01). Recommendations for future research offers suggestions for additional covariates to analyze. Implications for practicing school counselors as well as those who train, support, and evaluate them are also addressed. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]