In this chapter, a conservation of mass approach is used to develop a broad picture of the flow field in a glacier or ice sheet. Vertical velocities are shown to be downward in the accumulation area and upward in the ablation area, and horizontal velocities to increase with distance from the head of the glacier, reaching a maximum just below the equilibrium line. Conservation of momentum is then used to calculate the variation, with depth of horizontal and vertical velocity. Effects of valley sides and laterally non-uniform basal boundary conditions on the flow field are explored. Next patterns of internal reflectors imaged by radar are shown to reflect variations in effective strain rate and ice fabric. The reflectors can also be used to measure vertical strain rates and sub-ice shelf melt rates, and to document changes in the flow field over millennial time scales. Drifting snow also affects the flow field in polar environments, leading to development of narrow accumulation zones along the margin, and thus to formation of ice-cored moraines somewhat upglacier from the margin. Finally, inhomogeneous bed conditions beneath ice sheets can lead to streaming flow.