Social support from parents serves as one of the primary influences of youth physical activity-related behaviors. A systematic review was conducted on the relationship of parental social support to the physical activity-related behaviors of youth. Four categories of social support were identified, falling under two distinct mechanisms--tangible and intangible. Tangible social support is divided into two categories: instrumental--purchasing equipment/payment of fees and transportation--and conditional--doing activity with and watching/supervision. Intangible social support is divided into the two categories of motivational-- encouragement and praise--and informational--discussing benefits of. The majority of studies demonstrated positive associations among selected measures of parental tangible and intangible social support and youth activity. Overall, parental social support demonstrated positive effects. Many studies, however, combine social support categories and/or respondents into composite measures, making it difficult to disentangle the specific effects of parents and the type of support provided. (Contains 3 tables.)