The present monograph is a detailed taxonomical description of the diatom flora of the Laptev Sea (Arctic Ocean). The Laptev Sea is one of the most important epicontinental seas of the Arctic, marked by large freshwater inputs. A total of 344 taxa (56 genera) was identified. The most diverse genera are Navicula (72 taxa), Pinnularia (27 taxa), Nitzschia (21 taxa), Cymbella (20 taxa), Eunotia (20 taxa), Fragilaria (20 taxa) and Achnanthes (16 taxa). In surface sediments, the most abundant species are typical marine planktonic diatoms, such as Chaetoceros spp., Fossula arctica, Fragilariopsis oceanica, Thalassiosira antarctica, Thalassiosira hyperboreae, and Thalassiosira nordenskioeldii. Approximately 60% of the diatoms found in the Laptev Sea are freshwater species which originally occur in lakes and rivers of the Siberian hinterland. These diatoms are transported into the Laptev Sea by five large streams.