Vacuum high-voltage insulation has been investigated for many years. Typically, electrical breakdown occurs between two broad-area electrodes at electric fields 100-1000 times lower than the breakdown field (about 5000 MV/m) between a well-prepared point cathode and a broad-area anode. Explanations of the large differences remain unsatisfactory, usually evoking field emission from small projections on the cathode that are subject to higher peak fields. The held emission then produces secondary effects that lead to breakdown. This article provides a significant resolution to this long standing problem. Field emission is not present at all fields, but typically starts after some process occurs at the cathode surface. Three effects have been identified that produce the transition to field emission : work function changes; mechanical changes produced by the strong electrical forces on the electrode surfaces; and gas desorption from the anode with sufficient density to support an avalanche discharge. Material adsorbed on the cathode surface increases the work function of the metal, leading to a much higher threshold for field emission and higher breakdown fields. Localized regions of lower work function can be produced on the cathode by the transfer of microparticles from the anode and by stripping small areas of the cathode. The regions of low work function then serve as the source of enhanced field emission, leading to secondary effects which produce breakdown. Gas desorption is produced at an unconditioned anode as the voltage is increased. None of these effects are significant for a point cathode opposite a broad-area anode, but account for much of the large difference between microscopic and macroscopic breakdown fields. Careful surface preparation of electrodes increases the work function and reduces the number of weakly bound microparticles. Experiments designed to optimize these two different effects have led to electric fields as high as 90 MV/m at a 1 mm gap and 50 MV/m at a 4 mm gap, with no measurable field emission with plane-parallel electrodes made from copper, aluminum, titanium, and niobium. These fields, with no field emission or sparks during the conditioning phase, are comparable to the highest fields ever reached between plane-parallel electrodes of the same gap by any traditional conditioning method. The experimental results have been applied to operation of the electrostatic deflector of the Chalk River superconducting cyclotron. It has been used reliably for thousands of hours at fields up to 15 MV/m at a 5 mm gap, usually with no field emission. Experiments have also demonstrated that there is little enhancement in field emission at gaps up to 4 mm and that the only total-voltage effect for these gaps is from reduced thermal stability of the anode as the power density from the electrons increases with increasing voltage.