Proper wall conditioning has turned out to be an essential element for achieving the highest possible plasma performance in present day fusion devices. The main issues are controlling the generation of plasma impurities, liberated by plasma - surface interactions, and controlling the recycling hydrogenic fluxes. The underlying mechanisms are discussed in this paper. The paper presents a review of the different wall conditioning methods. It focuses on low-Z wall coatings (beryllium evaporation, boronization, siliconization, lithium pellet injection) and on helium glow discharge cleaning and assesses their effects on fusion plasmas. New wall conditioning concepts, compatible with steady-state magnetic fields, are discussed in view of future large devices with superconducting coils.