While sea ice will form whenever the temperature is sufficiently low, the snow cover on the ice will form only if the ice is already in place, if there is precipitation and if it is below freezing. The snow cover can either accelerate or retard the sea-ice growth, but which trajectory is followed depends very much on the sequence of weather events producing the ice and snow cover. Two key equations, linked by snow depth, density and thermal conductivity, govern the most important aspects of the interplay between the snow and ice during the winter months, but their simplicity belies the myriad of trajectories that can be taken by the snow sea-ice system. A second set of equations linking the surface energy balance and albedo dominate in spring and summer as the sea ice melts. As the sea ice diminishes due to global warming, understanding the interplay of the snow and ice takes on new importance because snow is one of the few potential factors in the sea-ice system that could slow the rate of decline.