A review of the studies being conducted by various laboratories and collaborations in order to determine and optimise the next generation of particle accelerators for physics at the high energy frontier beyond HERA1) at DESY, LEP2) and LHC3) at CERN, SLC4) at SLAC and the TEVATRON5) at FNAL is presented. The relative advantages of the Very Large Hadron Colliders, Electron Positron Colliders and Muon Colliders are compared pointing out their main challenges and key issues both in beam dynamics and technology. The present status and future plans of the studies are summarised outlining the research and development of key components and their tests in ambitious test facilities. Finally, the schedules presently assumed and the possible scenarios for the post-LHC-era around 2010 are presented
