The last two decades have seen significant advances in understanding the cycling of carbon and nutrients in ecosystems characterized by seasonal snow cover. This paper reviews and summarizes work on the interactions between seasonal snow cover, soil physico-chemical characteristics, biological activity, and plot- to ecosystem-scale carbon and nitrogen cycling. The magnitude of winter biogeochemical activity is considerable. For example, including these winter fluxes into annual estimates of net ecosystem exchange reduces annual carbon uptake by 50% or more in many ecosystems. The primary climatic control on these fluxes is the amount and timing of precipitation, especially the formation of a consistent seasonal snow cover. Consistent snow cover limits frost damage and controls both the timing and amount of liquid water in soil and the availability of labile carbon substrates. Together, liquid water and labile carbon control the magnitude of in situ activity, exchanges of CO2 and trace gases, and export of dissolved nutrients. The importance of snow cover to biogeochemical fluxes has led a renewed interest in how spatial variability in vegetation structure influences snow cover through shading, wind sheltering, and interception. Changes in snow cover associated with ongoing changes in both temperature and precipitation have the potential to profoundly impact the soil environment during winter and spring with unclear effects on annual and longer-term patterns of carbon and nitrogen cycling.