Students with extensive support needs (ESN) are a heterogenous group of students with the most pervasive and ongoing support needs who typically receive special education services under the categories of autism spectrum disorder (ASD), intellectual disability, or multiple disabilities and often qualify to take their state's alternate assessment (Taub et al., 2017). Students with ASD who have ESN may have elevated support needs for social behavior (Jang et al., 2011; Matson et al., 2011; Shogren et al., 2017). Although there are several evidence-based practices to support the behavioral needs of students with ASD who have ESN (Steinbrenner et al., 2020), educators often have difficulty implementing these practices with fidelity (Brock et al., 2014; Morrier et al., 2011; Robertson et al., 2020). School-wide Positive Behavioral Interventions and Supports (SWPBIS) is an evidence-based framework to support the social and behavioral needs of all students with evidence-based practices, data-based decision making, and systems to support teacher implementation fidelity (Horner & Sugai, 2015; Sugai & Horner, 2006, 2009). However, students with ASD who have ESN are not consistently included in SWPBIS (Kurth & Enyart, 2016; Kurth & Zagona, 2018; Walker et al., 2018). Check-in/Check-out (CICO) is an evidence-based intervention commonly used as a Tier 2 behavioral support within a SWPBIS framework (Conley et al., 2018; Maggin et al., 2015). CICO is effective for K-12 students without disabilities and students with high incidence disabilities (Maggin et al., 2015). The purpose of this study was to examine the effects of traditional or adapted CICO on the adherence to schoolwide expectations and challenging behavior of students with ASD who have ESN. Results of this single-case, multiple baseline across participants design study indicated there was a decrease in challenging behavior for two of the four participants when adaptations were made to the standard CICO protocol. Additionally, educators, students, and parents found CICO feasible and socially valid. Limitations, implications for practice, and suggestions for future research are discussed. [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.]