abstract: Digital libraries is an umbrella term that encompasses the automation of library services, online catalogs, information retrieval systems, multimedia databases, data archives, and other Internet-facing collections of digital resources. Clifford Lynch has played pivotal roles in the technical development, institutionalization, policy, practice, and dissemination of digital libraries for more than 40 years. Beginning with his foundational role in building MELVYL for the University of California in the early 1980s—the first Internet-native online open access library catalog—through his convening roles in open access and open data in the twenty-first century, his career is marked by multiple milestones of innovation. Clifford Lynch's career has traced the trajectory of digital libraries and knowledge infrastructures. Over the course of these 40 years, research libraries have faced four categories of challenges: invisible infrastructure, content and collections, preservation and access, and institutional boundaries. These challenges have become yet more complex in an era of open access, open data, and evolving regimes of intellectual property and scholarly publishing. As the digital library communities have merged and diverged over this time span, we collectively face challenges for at least the next 40 years to sustain access to current resources while growing the next generations of digital libraries and librarians.