The present doctoral dissertation focused on the impact of outdoor environments on children's physical activity and active free play. Three studies investigate the environmental influences within children's microsystems: school gardens, adventure playgrounds, and conventional playgrounds, in order to systematically improve the salutogenic (health-promotive) design and effectiveness of built and natural environments to promote children's physical activity, health and well-being (Antonovsky, 1987, 1996). First, Children's physical activity while gardening: Development of a valid and reliable direct observation tool (Chapter 2), the Physical Activity Research & Assessment tool for Garden ObservatioN (PARAGON), a direct observation tool was developed to operationalize children's physical activities, movement, and postures, while gardening at school. The study assessed the test-retest and inter-rater reliability of PARAGON as a measure of children's physical activity levels and movement in school gardens and assessed the validity of the physical activity codes. This valid and reliable direct observation tool may help to increase effectiveness of garden interventions that aim to improve children's health and well-being and can be used by community organizations to translate research into practice and practice into research. The second study, Children's physical activity in free-living outdoor environments (Chapter 3), a within-subjects design exploring two playground types, (conventional and adventure) and their influence on children's physical activity, while outdoors. This study had two purposes: a) to measure and compare children's physical activity, during active free play, in two playground types (conventional versus adventure), using two measures (accelerometry and direct observation); b) to explore the role of gender and playground types on physical activity. This study contributes to the literature exploring the physical environmental influences on children's physical activity, while engaging in outdoor, active free play, which will be important in the need to collect national data on children's active free play levels. In addition, this was the first study to look at adventure playgrounds effects on physical activity. And finally, the study filled a gap in out-of-school time (OST) research, exploring how the OST physical outdoor environments may promote children's physical activity. The third study, Adventure playgrounds and active free play: The role of environmental design in play behavior types, social interactions, and gender-inclusive space use (Chapter 4), also utilized a within-subjects research design to examine the effects of playground type (adventure playground compared to conventional playground) and gender on three components of active free play: 1) play behavior types; 2) social interactions and; 3) gender-inclusive space use. Results showed that in the adventure playground, both boys and girls engaged in a greater variety of active free play behaviors, engaged in more time in pro-social interactions, and spent less time in conflict interactions than in the conventional playground. Though the conventional playground was hypothesized to be a more 'gendered space' compared to the adventure playground, playground type was found to have no effect on the third dependent variable, gender-inclusive space use. Though the history of adventure playgrounds is long to our knowledge, this is the first quasi-experimental study to examine the effects of an adventure playground on children's active free play components (play behavior types, social interactions, and gender-inclusive space use) compared to a conventional playground.