"Terra Cognita" surveys the current landscape of archival research and the experiences of emerging scholars seeking to navigate it. Drawing on data from the Council on Library and Information Resources' (CLIR's) Mellon Fellowships for Dissertation Research in Original Sources, the report takes an in-depth look at how the conditions and practices of original source research have changed in recent decades and what communities invested in cultural heritage research can do to better support new scholarship in this evolving context. Part one of the report presents an assessment of the fellowship program based on data that fellows submitted in their final reports to CLIR between 2003 and 2015. In part two, program mentors Elliott Shore and Ryan Kashanipour share observations based on their work with the fellows. Part three presents broader perspectives on original source research, prompted by discussions at a meeting hosted by CLIR in January 2016. The volume closes with an afterword by Charles Henry that contemplates the intellectual and contextual challenges of conducting original source research today. Throughout the volume are brief reflections by past fellows about their research experiences and how the fellowship has influenced their careers. Contents include: (1) Foreword (Nicole Ferraiolo); (2) Ongoing Challenges for Research Using Primary Sources: An Analysis of Mellon Dissertation Research Fellow Reports (Lori M. Jahnke and Amanda Watson); (3) Not in This Alone: Toward an Interdisciplinary Community of Scholars and a Critical Archival Practice (Elliott Shore); (4) Mentoring and the Challenge of the Humanities (R. A. Kashanipour); (5) Renegotiating the Archive: Scholarly Practice in a Digital Age (William G. Thomas, III); (6) Reading the Materiality of the Archive (Michael F. Suarez); and (7) Afterword: This Vital Conversation (Charles Henry). Individual chapters contain references.