Open-source software (OSS) project are now widely used across academia, industry, government, and non-profits, but many of these projects are heavily or extensively developed and maintained by volunteers. In this project, we investigate the visible and invisible work that project maintainers do to support their projects and communities, as well as issues of burnout and maintainer sustainability. This is a collaborative project involving qualitative and quantitative researchers from UC-Berkeley, UCSD, UConn, and CNRS, and was funded by the Sloan and Ford Foundations as part of a large cohort of grants on OSS sustainability. The research involves interviews with project maintainers and contributors from a wide variety of projects, as well as analyses of projects’ code repositories and communication platforms. The goal of the research is to better understand what project maintainers do, the challenges they face, and how their work can be better supported and sustained.