In this chapter, we find that snow accumulation and its transformation to ice lead to stratigraphy that persists for thousands of years and can be used to date ice. A perturbation analysis is then used to show that net balances are sensitive to summer temperature in continental areas and to both winter balance and summer temperature in maritime ones. Radiation balance could play a role in either environment. Dynamic thickening or thinning, and terminus advance or retreat are found to be closely linked. A rapid advance accompanied by commensurate thinning does not change mass. However, an advance may result in rapid terminus melting and retreat. Calving and bottom melting are also important mass balance components. On tidewater glaciers calving can lead to retreats unrelated to climate. Calving is a dominant process of mass loss on ice sheets. Bottom melting is important on some valley glaciers and ice shelves. Variations in atmospheric circulation patterns may result in asynchronous mass-balance patterns on glaciers only a few hundred kilometers apart. Finally, estimates of global mass balance, and their contribution to sea level rise are summarized.