Polar oceans are distinct from other oceanic environments in a number of ways, but the presence of sea ice is a major habitat difference. Sea ice affects polar microbial communities by limiting light penetration into the upper ocean and by providing a unique sea-surface habitat (Figures 1 and 2). Sea ice serves as a support matrix for a diverse and dynamic assemblage of microbes, including phytoplankton and prokaryotes, often referred to as the sea-ice microbial community, or SIMCO. Growth of ice-associated microalgae can lead to extreme carbon enrichment in the ice, fueling microbial production and providing a food supply for herbivorous metazoa. Brine exclusion during the formation of sea ice, coupled with summer melting, contributes to significant and persistent water-column stratification. This is especially the case in the Arctic Ocean where permanent ice cover constrains wind mixing and the land-locked geography of the basin restricts exchange with lower-latitude waters. Other characteristics of polar oceans, such as low temperature and intense seasonal variation in primary production and carbon flux, are often more extreme than in other oceanic habitats.