The Climate Ecological Observatory for Arctic Tundra (COAT), a key component of the Svalbard Integrated Arctic Earth Observing System (SIOS), is the main agent for monitoring the state of terrestrial ecosystem. However, the direct assessment of reindeer-plant interactions is still not part of their current monitoring efforts, despite that grazing influences the structure and productivity of Svalbard vegetation communities, and in turn, alters ecosystem functioning, including nutrient cycling. Our preliminary analysis of Svalbard reindeer diet reveals considerable discrepancy in the estimated proportion of ingested plant species using micro-histology of plant fragments, compared to modern DNA metabarcoding and stable isotopes methods, thus calling for the rigorous assessment of these three methods. PIECEMEAL will bring together six internationally regarded experts from five different countries in each of the three diagnostic techniques, with colleagues in both the Norwegian Polar Institute (NPI) in Ny-Ålesund and UNIS in Longyearbyen. They will take advantage of a rare opportunity of working on the same physical samples (32 female reindeer to be culled in 2022), to provide a cost-efficient and ultimately non-invasive way (based on faecal sampling), to reliably characterise Svalbard reindeer diet. These data will allow better mechanistic models of both reindeer population growth, Svalbard plant community dynamics, and tundra ecosystem processes, as the climate continues to warm. Faecal and tissue samples are also available from reindeer in Ny-Ålesund since 2010. Thus, dietary studies here will reinforce a major component of the SIOS Terrestrial Ecosystem flagship programme by providing a non-invasive tool for diet and trophic interaction monitoring. Furthermore, COAT will be able to directly compare the diet of the ‘coastal’ Ny-Ålesund reindeer, grazing a sparser vegetation cover, and the ‘inland’ reindeer population on the better vegetated Nordenskiöld Land.