School-based mental health programs are a powerful mechanism for increasing access to mental health care for the estimated 50% of children who do not receive needed interventions. However, services provided in schools rarely follow evidence-based practice (EBP). Identifying determinants of provider attitudes would illuminate strategies for increasing EBP usage. Both supervisors and peers influence school mental health provider (SMHP) attitudes, but the relative importance of these two sources is unclear. Source influence may be related to credibility, as receiving information from a highly credible source leads to more significant attitude change. SMHPs recruited online through direct contact, Prolific, or Amazon Mechanical Turk were randomly assigned to groups based on the source of information regarding the intervention -- peer opinion leader, supervisor, or a professional organization (as a control group). SMHPs imagined that their identified source shared information about the Resilience Builder Program® (RBP), a transdiagnostic, strengths-based group cognitive behavioral therapy intervention. Participants completed questionnaires regarding their source's credibility, attitude toward EBP, perceptions of the RBP's acceptability/efficacy, commitment to implement the RBP, and demographic information, as well as change readiness among SMHPs, school leaders, and the external community. After data integrity checks, the final sample included 183 participants who were mostly White (74.86%) and female (79.78%), with a mean age of 38.78 years (SD = 10.47). The role of demographic characteristics was examined using one-way ANOVAs and bivariate correlations. Peer and supervisor influence on attitudes and intentions to use the intervention was analyzed using conditional process analyses using the SPSS PROCESS macro. Most demographic characteristics were not related to attitudes toward EBP in general or the RBP, but those with a bachelor's degree indicated more positive perceptions of the RBP than those with a more advanced degree. There were no source group differences in credibility and thus no differences between groups in paths involving credibility as a mediator. Despite this lack of group difference in credibility, receiving information from a supervisor (relative to a peer) led to more positive attitudes, and therefore higher intention to use the intervention. Collapsing across sources, credibility significantly predicted perceived acceptability/efficacy, and in turn predicted implementation commitment. No type of contextual change readiness moderated the relation between attitude and implementation commitment, but external community change readiness was directly positively related to implementation commitment. Data from this study support the importance of credibility in shifting attitudes and that any credible source may be effective. Supervisors may be more impactful than peers regardless of similar levels of credibility. Among contextual factors, external community readiness for change had the strongest influence. Overall, school-based EBP implementation is likely best supported by providing information from credible sources (especially supervisors), specifically targeting SMHP attitudes, and engaging the external community. [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://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]