Summary: Dunes (in the desert, or in rivers) are ridges of sand which are built by the erosive action of wind (or water) over the underlying, mobile substrate. Drumlins are small hills which are similarly formed by the erosive action of ice sheets over mobile sediments, particularly during ice ages. Both types of bedform are caused by an instability in the couples system which relates bed evolution to the shear stress exerted by the wind, water or ice which flows over it. Mathematical models predict this instability, but the development of realistic nonlinear models is difficult because the flow separates behind the developing bedforms. Simplified models which include such separation in a realistic way have not yet been developed.