Purpose: The purpose of this mixed-methods study was to examine the effects of Positive Behavior Interventions and Supports (PBIS) on students in transitional kindergarten (TK) through second grade before implementation and after implementation of PBIS at platinum level elementary schools. Methodology: A mixed-methods study will be used to examine the effects of PBIS on students in TK through second grade before implementation and after implementation of PBIS at platinum level elementary schools in southern California. The quantitative method was used to gather archival data on pre PBIS implementation and post PBIS implementation to determine if there was a difference in student behavior incidences. The qualitative method was used to gather data from the four platinum level school site administrators on pre PBIS and post PBIS implementation to find if they believe student behaviors were impacted. The qualitative data was gathered during semi-structured interviews by the researcher. Findings: Findings from the quantitative and qualitative research showed a significant difference in the number of referrals written pre- implementation to post implementation of PBIS at four TK-2nd Platinum level elementary schools. All four site administrators believe that PBIS has positively impacted behaviors on their school sites. Conclusions: Based on the findings from this study and the increase in the number of referrals written it can be concluded that PBIS is providing a comprehensive and consistent system of recording data that is then used to help support student needs. [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.]