This brief and accompanying Funder's Sustainability Assessment Tool is intended to help foundation leaders address the challenges of sustainability. The brief begins with a framework for thinking about sustainability, introduces an assessment tool funders can use in a variety of ways to support sustainability in their grantmaking, and concludes with guidance on how to use and adapt this tool for particular purposes. [An initial version of this tool was developed for the Jim Casey Youth Opportunities Initiative to support their ongoing grantmaking decisions.]