The ability to engage in classroom activities is associated with better academic outcomes (Downer et al., 2007; Ponitz et al., 2009), and characteristics of children can affect how a child is able to engage in classroom activities (McWilliam & Bailey, 1995; Kishida & Kemp, 2006). Yet, support from adults can enhance the engagement of children (Sutherland & Oswald, 2005; de Kruif & McWilliam, 1999). To date, researchers have focused on the interactions between adults and typically developing children in classrooms. Limited information is known concerning how adults support the engagement of children with autism spectrum disorder (ASD) in preschool classrooms. The present study will examine 1) the patterns of adult participation and child engagement in classrooms that serve children with ASD; 2) the associations between child engagement and adult participation; 3) if autism severity moderates the relationship between adult participation and child engagement; 4) if language ability moderates the relationship between adult participation and child engagement; and 5) if problematic behavior moderates the relationship between adult participation and child engagement. Participants included 190 preschool aged children diagnosed with ASD enrolled in the Autism Spectrum Disorders Treatment Comparison Study. Each participant was videotaped 30 minutes during center time activities by research staff. Through using an ecobehavioral assessment new variables were created: Adult Participation and Child Engagement. Children were actively engaged for more than 70% of the time during center time. Adult participation levels were associated with child engagement; specifically, no adult participation was associated with higher levels of child engagement. Children with less severe autism, more language ability, and less problematic behaviors had higher levels of engagement in activities, and were more likely to be engaged when there was no active adult participation. Adult participation was associated with higher levels of engagement for children with more severe forms of autism. For children with less language ability, active adult participation was associated with low levels of engagement. Finally, problematic behavior was correlated with lower levels of engagement, yet these children had higher levels of engagement when adults were not participating. Implications for practice and future research needs are 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://www.proquest.com/en-US/products/dissertations/individuals.shtml.]