Organic matter is of primary importance to the sustainability of long-term site productivity in forest ecosystems. In boreal forests, organic matter accumulates at the surface as mor humus. This may represent a substantial portion of the total nutrient capital of a site, and its decomposition is essential for the short-term availability of nutrients for tree growth and long-term site fertility. However, organic matter accumulation at the soil surface can also effect the forest ecosystem by immobilizing nutrients making them unavailable for plant uptake, and by creating physical and environmental conditions that can impede seedling establishment and survival. Therefore, it is necessary to understand the processes of humus formation and decomposition in order to manage these soils in a manner that will maintain or improve site productivity. This paper provides an overview of (i) the composition of humus, (ii) the conditions in the boreal forest that result in the surface accumulation of humus, (iii) decomposition processes and (iv) the effects of humus on nutrient (especially nitrogen) availability. Questions relating to the detrimental role of surface organic matter accumulation, the effects of natural disturbances (e.g., fire) and harvesting disturbances on humus loss and accumulation and management practices that can maintain long-term site productivity will also be discussed.