Autism spectrum disorder (ASD) is a complex neurodevelopmental disorder characterized by deficits in social communication and pervasive repetitive behaviors (American Psychiatric Association [APA], 2013). Further, ASD is one of the fastest rising childhood developmental disorders, affecting 1 in 68 children (Christensen et al., 2016). In addition to the hallmark charactizations of this condition is a growing body of research (Lloyd, MacDonald, & Lord, 2013;Liu, Hamilton, Davis, ElGarhy, 2014;Staples & Reid, 2010) that suggests that individuals with ASD also demonstrate delays in the development of gross motor skills. Despite mounting evidence of delay, few interventions have targeted gross motor skills as an outcome (Staples, MacDonald, Zimmer, 2012). Three recent studies (Bremer, Crozier, & Lloyd, 2016; Ketcheson, Hauck, & Ulrich, 2016), amoung others, demonstrate the increasing awareness to this issue; however, continued theory-based research is needed to builded an effective motor intervention for children with ASD.The purpose of this parallel, convergent mixed methods design study was to test the validity and effectiveness of a fundamental motor skill (FMS) intervention for children with ASD that uses dynamic systems theory (DST; Newell, 1986). The intervention was based on intentional manipulations of task constraints, hereafter referred to as task modifications, to build FMS. This study incorporated both quantitative and qualitative data to understand not only how the task modifications work to influence the motor performance of children with and without ASD, but how potential changes in motor performance, or the perception of, was understood by parents. Furthermore, this study sought to understand how the intervention effected the daily lives of the individual iv and their family.Results revealed that a motor intervention based on dynamic systems theory may: (a) significantly improve gross motor performance of children; (b) provide an effective means to build motor skills in children with ASD; and (c) allow for a high level of engagement and successful practice. Furthermore, parent interviews suggest that there are number of barriers to physical activity, as well as many benefits. Futher, results suggest that the home environment may play a role in the gain made during a school based intervention; acting as an environmental constraint. Lastly, when looking at changes in motor performance compared to themes discussed by parents, data suggest that motor skills may play a role as a mediating factor in a child's physical activity level.Additionally, the results suggest that direct changes from increases in motor performance may have delayed indirect changes in other developmental skill and in the child's life.While data suggest significant improvements, a small sample size and the heterogeneity of ASD limit the overall generalizability of the intervention. Further research is necessary to fully understand the potential of an intervention based on task modifications. Moreover, while significant finding...