The earliest work on quantum tunneling in solid‐state physics, more than fifty years ago, dealt with electron tunneling through a vacuum barrier. But for the next half century we had no clear experimental demonstration of this conceptually simplest of tunneling phenomena. Spectroscopic and technological exploitation of quantum tunneling was developed only with solid tunnel barriers. Metal–vacuum–metal tunneling requires a gap held constant at a few angstroms. At such small distances—just a few atomic widths—it is extraordinarily difficult to control the gap size and insure that surface contamination layers or irregularities do not result in an unwanted contact across the gap.