The Environmental Stress Hypothesis (ESH) illustrates the underlying mechanisms of internalizing problems in children with Developmental Coordination Disorder (DCD), indicating that the relationship between DCD and internalizing problems could be influenced by numerous physical and psychosocial consequences. However, the potential pathways described in this conceptual framework have not been comprehensively examined. Furthermore, given that child development is a dynamic process, these pathways have not been investigated from a developmental perspective. In order to address these gaps in knowledge, this dissertation sought to advance our understanding of the ESH by examining the underlying mediating pathways connecting DCD and internalizing problems in three age groups: early childhood, late childhood/early adolescence, and young adulthood. Study 1 showed that preschool children at risk for DCD (rDCD) experience more internalizing problems than typically developing children. However, physical activity and BMI do not mediate the relationship between rDCD and internalizing problems. Overall, our findings confirm that rDCD and internalizing problems co-occur in early childhood. Nevertheless, as there is no mediation of physical activity or BMI, the underlying mechanisms may be more related to other psychosocial outcomes (e.g., self-concept or perceived social support), suggested in the ESH. Study 2 examined school-aged children and included global self-worth, one of the psychological outcomes identified in the ESH, to address one of the limitations in Study 1. Findings support the ESH by showing a sequential mediating pathway from probable DCD (pDCD), through physical activity/BMI and global self-worth, to self-reported internalizing problems. Sex was found to moderate the underlying mechanisms of internalizing problems, altering the pathways from pDCD to internalizing problems. Study 3 was conducted to test the full ESH in emerging adults. Results support the mediating effects of psychosocial well-being, including stress, global relationships, perceived social support, and self-concept, on the relationship between poor motor coordination and self-reported psychological distress in young adults. However, in this age group, physical inactivity and higher BMI, did not mediate the relationship between motor coordination and internalizing problems. In conclusion, this dissertation highlights the co-occurrence of motor difficulties and internalizing problems across three developmental stages. The underlying mechanisms of internalizing problems may differ by age and sex. It is also worth noting that compared to physical health, psychosocial well-being may play a more important role as a mediator in the relationship between motor coordination and internalizing problems. / Thesis / Doctor of Philosophy (PhD) / Developmental Coordination Disorder (DCD) or poor motor coordination has been linked to the higher levels of internalizing problems, such as anxiety and/or depression. However, the underlying mechanisms remain unclear. Although the Environmental Stress Hypothesis (ESH) provides a theoretical framework illustrating that physical and psychosocial consequences of DCD may play important roles, this model has not been comprehensively examined. Therefore, this dissertation tested different components of the ESH in three age groups (i.e., preschool age, school age, and young adulthood) in an attempt to provide a better understanding of why motor difficulties cause more internalizing problems from a developmental perspective. Findings confirm that children with DCD and adults with poor motor coordination are at greater risk for internalizing problems, and that perceptions of self may play a more important role explaining the relationship. However, the potential pathways from motor difficulties to internalizing problems may differ by sex and age. In summary, it is recommended that, in order to prevent or improve mental health problems, interventions should target improvements in self-concept, such as self-esteem, through participation in physical activity or weight control, while taking into account sex and age.