In the Arctic tundra several different species of herbivores co-exist, with potentially large effects on tundra ecosystem functions and dynamics. Understanding the herbivores trophic interactions therefore provides essential knowledge, especially at a time when the Arctic terrestrial ecosystem is under radical changes due to the continued climate warming. In the first part of my thesis, I wanted to establish how the detectability of DNA changes along the digestive tract of ptarmigans, with my primary objective to test the hypothesis that fecal samples is the most representative sample type and thus can provide the most comprehensive diet analysis. DNA metabarcoding was used to compare four different digestive samples (crop, gizzard, fecal samples from the large intestine, and fecal samples from the ceca). On overall the fecal samples (from the large intestine and the ceca) combined contained the highest percentage of the total detected dietary items (90%), as compared to the samples from crop and gizzard combined (67.5%). However, when excluding fecal samples from the ceca, approximately 13% fewer dietary food items were detected from the fecal samples from the large intestine (79.7%) than from the crop and gizzard samples combined (93.2%). Despite this decrease, fecal samples from the large intestine are still a good representative sample type for dietary analysis in ptarmigans, as most of their diet diversity were captured. In the second part of my thesis, DNA metabarcoding was used to study diet composition and overlap of five sympatric herbivores in Finnmark, Norway. The herbivores included in this part are ptarmigans (Lagopus spp.), reindeer (Rangifer tarandus), hares (Lepus timidus), moose (Alces alces) and rodents. Diet analysis of the herbivores field-collected fecal samples, using the vascular plant primers G/H, detected that the genus Betula, shrubs and ferns, were the most common diet categories, followed by mosses, graminoids and forbs. However, the diet composition of individual species varied. For ptarmigan and reindeer, Betula was the most dominating diet item. In hare and rodent diet, shrubs were dominating. While ferns were dominating in the moose diet, followed by shrubs. Within shrubs, four dietary items were detected with higher frequency of occurrence. The dwarf shrub genus Empetrum was detected in ptarmigan and rodent diet. The dwarf shrub species Vaccinium ulignosum and genus Vaccinium were detected in ptarmigan, reindeer, hare and rodent diet. Whilst the shrub tribe Saliceae were detected in ptarmigan, hare and moose diet. Thus, by successfully using field-collected fecal samples, multiple shared MOTUs between the herbivores were detected. These results indicate that high niche overlap exist between the studied herbivore species, but to a lesser extent when including moose.