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 optimize environments that encourage physical activity across the life course, we argue that researchers in this field to think beyond their 'square' i.e., the target group, setting and physical activity behavior with which they work. We suggest that researchers working in this field need to better understand systems theory to appreciate that a change to one part of a complex system, can positively and negatively impact other parts of the system. An understanding of systems theory would help minimise unintended negative consequences to other population sub-groups or other types of physical activity from the implementation of our research findings. By so doing, a more comprehensive set of research, practice and program-related activities may emerge that will advance physical activity research and practice and, in so doing, improve population health across the life course.