Given their portability, mobile devices allow users to access information, record field observations, or search databases on-site to identify plant and animal species present in natural, outdoor settings. This paper presents five empirically based design guidelines for mobile learning outdoors that our research team used to design a mobile app to help families learn about tree life cycles. We link theory to practice by illustrating how the five guidelines were applied in the context of an outdoor mobile learning project called "Tree Investigators." [For the complete volume, "Educational Media and Technology Yearbook, Volume 39," see ED594284.]