This chapter summarizes the results of a series of investigations on the ecology and diversity of fungi associated with the decomposition of Arctic and Antarctic plant remains, with special reference to dead materials of moss and willow that are major components of cold regions. Fungi are major components of the diversity of soil organisms in polar regions, and they play central roles in the decomposition of plant remains that control carbon and nutrient cycling in soil. Mosses are major components of terrestrial ecosystems of the Arctic and Antarctica, and contribute importantly to the crucial budgets of carbon and nutrients via primary production and concomitant decomposition and accumulation of moss tissues in soil. The contents of organic chemical components and nitrogen were assessed within the arctic and Antarctic moss profiles. The amount of holocellulose relative to recalcitrant compounds classified as acid unhydrolyzable residues decreased downward from the upper to the lower layers of two arctic moss profiles.