Abstract Colliding beam teahniques require ultra-high vacuum and particle currents and densities exceeding substantially those available in present-day accelerators. Accumulation of pulses of accelerated particles in storage devices is commonly considered a way of obtaining the particle numbers necessary. Design proposals for such devices and knowledge on the problems involved are due mainly to groups of the M.U.R.A., and of Princeton University, and to groups in Moscow. The principal difficulties arising are: the limitations to phase space compression imposed by Liouville's theorem, and gas scattering and other statistical effects tending to reduce density and lifetime of beams. In the case of relativistic electrons, orbit radiation helps to overcome these difficulties, and a storage ring system designed for the study of e-e interactions will be tested at Stanford University in the near future. Colliding beam experiments with protons, in conjunction with recently completed Multi-GeV proton synchrotrons, might be feasible if beam stacking can be effected with maximum phase space economy. A low-energy model study planned at CERN is hoped will contribute to answering the question of feasibility.