This dissertation explores the functioning and participation of children with autism spectrum disorder (ASD) in their natural environments, with a focus on the preschool years. Chapter 1 provides the context for this dissertation through introducing the theoretical frameworks that were used, exploring function-based measures, and social communication functioning. Chapter 2 is a qualitative evidence synthesis of the literature to identify the cultural factors that are associated with parents’ engagement in their children’s healthcare services. This chapter highlights the challenges that parents of children with ASD faced and the importance of health providers to be culturally competent in family-centered care. Chapter 3 is a descriptive study that aims to explore the participation patterns of preschool children with ASD at home and in the community. The association of environmental factors and social communication abilities on children’s participation are also explored. This chapter emphasizes that children with ASD participated in a variety of activities at home and in the community. It also highlights the importance of supporting parents to engage their children in activities and the need for community to support their participation. Chapter 4 examines the stability of a function-based categorization system – the Autism Classification System of Functioning: Social Communication (ACSF:SC). Children’s social communication abilities were rated using the ACSF:SC at baseline and six months. This chapter indicates that rating of social communication abilities can vary over time. Therefore, frequent assessment using the ACSF:SC is recommended for accurate identification of the child’s current social communication abilities. Chapter 5 discusses the clinical and research implications of this dissertation work. It also discusses knowledge translation opportunities and potential future work. / Thesis / Doctor of Philosophy (PhD) / This thesis consists of a series of studies examining factors associated with the abilities of children with autism spectrum disorder and their participation in daily activities. The first study provides a review of the literature examining the association between cultural backgrounds of parents and their engagement in autism child services. The second study explores the patterns of engagement in childhood activities for preschool children with autism using parent-reported measures. This study also examines the associations between environmental factors and the social communication abilities of a child and their participation in childhood activities. The third study examines the social communication abilities of children with autism using a new approach to describing levels of ability, rather than deficits. This study looks at the social communication abilities of preschool children with autism over six months. The importance of this work is that it adds to the literature by generating new knowledge on the abilities of children with autism, rather than their deficits. It also emphasizes the need for support in healthcare settings and in the community to improve these children’s participation.