This chapter focuses on Pythium spp., which are the most common genus of Oomycetes in polar regions, although there are a few reports on Peronospora and Saprolegnia. Oomycetes, synonyms Oomycota and Peronosporomycetes, are fungus-like microorganisms belonging to the eukaryotic kingdom Chromista. Oomycetes and true fungi resemble each other in their gross structure, and comprise a mass of branched hyphae. In polar regions, Pythium spp. can cause snow rot on mosses. Pythium is the most cosmopolitan genus among Oomycetes, and holds more than 200 species through the world. Pythium polare is the most common Oomycetes in polar regions which has a bipolar distribution. This species is heterothallic, which requires antheridial and oogonial isolates to produce oospores—even in between the Arctic antheridial isolate and the Antarctic oogonial isolate. Pythium polare is able to infect Sanionia moss and cause brown discoloration. P. polare is also isolated from the non-symptomatic moss and fresh water.