A dramatic improvement in energy gain per unit cost is mandatory for the future of very high energy accelerators, for which RF-acceleration would be replaced by new tech-niques. Several schemes have been proposed (laser driven, beam driven accelerators) and tested. A critical review of these schemes and experimental results is presented, with considerations on the most promising techniques and the effort still needed. Important parameters for high luminos-ity (efficiency, emittance preservation, ...) are discussed
