With the advent of the super-tip gas field ion source very favourable conditions for generating ion nano-beams have been provided. The basic physical principles for this kind of source are described As a consequence of a very small source emittance of ε ~ 10-24 m2sr ultra-bright rare gas ion beams of about 1016 A/m2.sr can be produced. As a further consequence, very narrow probe sizes are obtainable, of dp< 1 nm, with spatial resolutions far below the classical limits of light-optics and even below the novel stimulated-emission-depletion microscopy [STED (microscopy) = stimulated emission depletion (microscopy)] Th. A. Klar, S. W. Hell, Opt.Lett. 24 (1999) 954-956]. Various ion-nanobeam applications are envisaged.