Plasma wakefield acceleration has made remarkable progress in the last ten years. Experiments at SLAC have shown that an energy gain by trailing electrons of 42GeV in only 85cm of plasma is possible. This corresponds to an average accelerating gradient larger than 50GeV/m sustained along a meter-scale plasma. These results were possible only because ultra-relativistic, short (~100fs), high peak current (~10kA) electron bunches, focusable to ~10µm transverse size, are available at SLAC. However, these bunches only carry ~60J, thereby limiting the ultimate energy gain by a trailing bunch. Therefore this electron beam driven plasma wakefield scheme will have to resort to staging to reach the energy frontier (0.5-1TeV).