Check-In/Check-Out (CICO) is a widely implemented, evidence-based Tier 2 behavior intervention used primarily in schools implementing Positive Behavioral Interventions and Supports (PBIS). The purpose of this national extant study was to examine implementation patterns using an extant database of 24,425 students within 1,766 schools across 40 U.S. states using a data-decision system for entering and reviewing CICO data. We used descriptive analyses and a Poisson regression model to examine patterns of implementation and identify predictors of student participation in CICO. Findings showed that the most common months for students to begin participating in CICO were September and October (elementary and middle schools) and October and November (high schools). The average number of students supported in CICO in elementary schools was 13.12, middle schools was 17.90, and high schools was 8.43. In addition, several variables were found to be significantly and positively associated (percentage of students receiving free and reduced price lunch [FRL], school size, schools in towns, and Tier 2 PBIS implementation fidelity) and others negatively associated (percentage of minoritized students, high schools, schools in cities, and rural areas) with student participation in CICO. Implications for how school teams can scale up CICO to support more students early in the school year and improve student participation in CICO are discussed. [This paper will be published in the "Journal of Positive Behavior Interventions."]