Seed libraries are institutions that support the creation of semi-formal seed systems, but are often intended to address larger issues that are part of the “food movement” in the global north. Over 100 SLs are reported present in California. I describe a functional framework for studying and comparing seed systems, and use that to investigate the social and biological characteristics of California SLs in 2016 and how they are contributing to alternative seed systems based on interviews with 45 SL managers. (...) At a minimum, SLs function as new seed distribution institutions founded and overseen by dedicated, values-driven individuals and groups with goals including education, seed access, local adaptation, biodiversity conservation, community-building, and human health. Annually about 4776 people borrow seeds from, and 238 people return seeds to the SLs in this study, that operate through over 17,000 hours of work/year. These SLs distribute approximately 6456 packets of seed annually, mostly of commercial seeds from small seed companies, but some SLs emphasize local and culturally meaningful seeds. The significance of a 6% seed return rate depends on SL goals and can be investigated once appropriate indicators for those goals are identified and documented. Beyond distribution, the seed system functions accomplished by SLs differ, and all can have consequences for the processes shaping the diversity and adaptation of their crops. The SLs engaged in seed system functions beyond distribution are new forms of socially-motivated community science, poised to develop biological and social innovations reflecting their values and interests. (shrink)