Three regimes in the formation of a simple pinch are delineated. For a rapidly rising current, the formation of the pinch by a strong shock is first considered. A slug model is developed which, unlike the snowplough model, describes the radial structure between piston and shock. The final equilibrium pinch is described and radial profiles of density and temperature are obtained. Under the conditions of a strong shock, a weakly compressed pinch is formed in which the pinch radius is related to the wall radius and is independent of the current applied. By programming the current in time, regimes of singular compression and adiabatic compression can be obtained. A much more highly compressed plasma is produced. Such a current profile can readily be produced by a line storage energy source.