In this paper, the 21st Century School Fund and the Center for Cities and Schools at the University of California Berkeley provide a conceptual frame for the joint use of PK-12 public school buildings. There is a growing conversation about and demand for joint use as a way to provide services to children and families in convenient locations, improving opportunities for physical activity by increasing use of school recreational and outdoor spaces, leverage capital investments, and more. However, engaging in joint use, particularly intensive sharing of space or use by multiple parties, presents ongoing challenges to school and community leaders. This paper attempts to frame the basic challenges and opportunities for joint use to facilitate better conversations and planning for these type of collaborations. (Contains 20 endnotes.) [This paper was also developed by the Center for Cities & Schools at the University of California, Berkeley and was made possible with the support of the Healthy Eating Active Living Convergence Partnership.]