In order to fulfill its role in the botanical community, an herbarium must have its specimens processed, identified, and accessioned so that botanists can consult these materials during their research. Many herbaria, however, struggle to keep up with the influx of specimens. As a result, specimens may accumulate while awaiting further processing, leaving the institutions with what is known as a ‘backlog’; yet we know little about the content of such backlogs. In this study, we surveyed herbarium staff about their institution's backlog of unaccessioned specimens, focusing on the number and taxonomic lineage of backlogged specimens as well as the challenges associated with curating these specimens. We asked additional questions about unaccessioned bryophyte specimens in order to explore these collections in more depth. Eighty-seven staff members responded to our survey and 65 of these responses were complete and thus able to be analyzed. Approximately 92.0% of these staff indicated that their herbarium had a backlog with an average of approximately 30,372 unaccessioned specimens per herbarium. According to Index Herbariorum there were 3,426 active herbaria in 2020, and thus we estimate that ~104 million specimens are present in herbarium backlogs. While bryophytes only represent 9% of the accessioned specimens reported by our study participants, they are overrepresented in the backlog, comprising 16% of all unaccessioned specimens. According to our staff respondents, slightly more than half of backlogged bryophyte specimens are stored in field packets without labels, and approximately three-quarters of the unaccessioned bryophytes are not yet identified to species. In addition to describing the challenges associated with the integration of partially curated materials into an herbarium, staff also discussed other limitations. These obstacles included competing institutional priorities, labor shortages, and insufficient bryological expertise. Drawing on the answers provided by our survey respondents, we offer suggestions regarding how best to accession specimens currently in herbarium backlogs.