The built environment and physical activity agenda provides a unique opportunity for public health, physical activity and planning researchers to be front and centre of a movement aimed at creating healthier and more sustainable environments. However, in order to optimise environments that encourage physical activity across the life course, researchers in this field need to think beyond their "square" -that is, the target group, setting and physical activity behaviour with which they work. We suggest that researchers working in this field need a better understanding of systems theory to appreciate that a change to one part of a complex system can positively and negatively influence other parts of the system. An understanding of systems theory would help minimise unintended negative consequences to other population subgroups or to other types of physical activity from the implementation of our research findings. In this way, a more comprehensive set of research, practice and programme-related activities may emerge, which will advance physical activity research and practice, and improve population health across the life course.