This chapter shows that the biodiversity, their distribution, and ecological roles of snow molds in polar regions. Fungal species were less frequently recorded from the cryosphere than those of temperate zone despite the fact that major fungal taxa have already been found in the cryosphere. Snow molds are incited by many fungi belonging to various taxa that attack dormant plants, such as forage crops, winter cereals and conifer seedlings under snow cover. Pythium snow rot occurs in Japan, Iran, the United States, and Kola Peninsula in the European part of Russia in the Arctic. The cryoseston is defined as the community of organisms living on snow. Algae which grow on snow are named “snow algae”. Snow algae are major organisms in the cryoseston, and fungi that associate with snow algae are referred to as snow fungi. Moss vegetation plays an important role in primary succession in polar regions and are a key part of the terrestrial ecosystems in polar regions.