This dissertation research responded to needs identified by researchers to better understand factors that influence different domains of physical activity (leisure, transportation, occupational, and household) and sedentary behavior at different life stages. In the context of the Ecological Model of Active Living, the present studies contributed to filling specific gaps in the literature on the effect of parenthood on physical activity. The studies addressed gaps related to sex-specific effects on parents, number and age of children, and how parenthood might interact with psychosocial and environmental factors to impact physical activity. Participants were 909 women and 965 men, aged 20 to 57 years. Paper 1 examined components of the intrapersonal level of the Ecological model, assessing the effect of parenthood on physical activity while controlling for demographic and biological factors and environmental features. Paper 2 explored interpersonal Ecological Model factors to assess whether parent status moderated the association between six psychosocial variables and physical activity outcomes. Paper 3 examined components of the environmental level of the Ecological Model to assess whether parent status moderated the association between eight objectively measured and four reported built environment variables and physical activity outcomes. In general, parents and non parents had similar levels of physical activity and responded similarly to psychosocial and environmental factors. Key differences were found at the individual, interpersonal, and environmental levels that could be helpful in developing parent-targeted interventions and merit further investigation. At the individual level, parents reported higher levels of household activity and less sitting time than non-parents, indicating favorable effects of parenthood. Compared to parents with children aged 0-5, non-parents and parents with children aged 6-17 reported less household activity and more sitting time, indicating the latter may be appropriate physical activity intervention targets. At the interpersonal level, social support was found to be a stronger correlate of physical activity for parents than non-parents, implying that social support-based interventions may be particularly effective for parents. At the environmental level, evidence of a parental disadvantage in access to recreation facilities and walkable neighborhoods supports the use of more broadly-targeted policy approaches to ensure parent access to physical activity resources.