[en] The concept of plasma potential arises from the interaction of a plasma with a boundary. Due to the high mobility of electrons, a potential difference develops between the two so that a positive space-charge region, the 'sheath', shields the plasma from the boundary. Losses of ions at the boundary, however, means that shielding is ineffective unless ions enter the sheath region with a sufficiently high velocity (the 'Bohm criterion'). Since this ion flux cannot be generated by thermal motion, there is a potential variation within the plasma itself (the 'presheath'), which accelerates the ions towards the plasma edge. The potential difference between a plasma and a boundary has been exploited in a wide variety of plasma surface engineering applications. The surface of a substrate immersed in a plasma will be subject to bombardment by ions accelerated across the sheath which will not only heat the substrate but can also sputter atoms out of the surface, modify the properties of films deposited onto the surface or result in bombarding species being incorporated into the surface. While energetic ion bombardment can be supplied by directed ion beams, it is more easily applied uniformly over complex surfaces by biasing a substrate immersed in a plasma with an appropriate negative potential, either DC or rectified rf. This is a feature of ion assisted deposition processes, both PVD and CVD, ion assisted thermochemical diffusion processes, such as plasma nitriding, and, in the limit of high bias potentials (10-100 kV), Plasma Immersion Ion Implantation (PIII or PI³ - Trade Mark). This paper reviews some of the interesting and intriguing aspects of the behaviour of low pressure rf plasmas when large perturbations occur to the potential distribution described above. These observations have been made as part of our work over the last ten years on the use of low pressure plasmas and high energy ion bombardment to extend the range of applicability of plasma nitriding, in particular to lower the treatment temperature