Wakefield accelerators are under development in many laboratories worldwide. They bring the promise of a high accelerating gradient, orders of magnitude higher than current machines. The reduction in the overall length of the accelerators will pave the way to a wider use of such machines, for industrial, medical, research, and educational purposes. At the same time, all the equipment must be reduced as well, to keep the dimensions of the machine as small as possible. The two main challenges of the diagnostics for plasma accelerated electron beams are the ability to measure the 6D phase space properties with single shot techniques and the compactness to meet the requirements of a `table-top' facility.