Equal Channel Angular Extrusion (ECAE) has become a very popular tool for studying the evolution of microstructure and properties under severe plastic deformation. It is believed that the stress-strain characteristics are uniform in a cross-section of the billet and this uniformity of the stress-strain distribution ensures the uniformity of microstructure and mechanical properties in ECAE processed billet. However, some experimental data such as the fracture of the extruded billet, which is initiated at the inner surface of the sample, has caused doubts about uniformity of stress-strain distribution. This non-uniformity has been proved recently by Finite Element Simulation. In this paper the studies of the positive role of the applied back-pressure during ECAE are reviewed and the influence of a back-pressure on the uniformity of the stress-strain distribution, strain localisation, die corner filing, and the prevention of fracture is shown. The effect of back-pressure on grain refinement and improvement in mechanical properties is emphasized. The paper summarises our results from over seven years of work using a unique machine for ECAE with computer-controlled back-pressure and velocity of the backward punch.