The angular resolution obtainable with different techniques and across the electromagnetic spectrum. The approximate domain in which diffractive X-ray optics are potentially of interest is indicated by an ellipse. Shown for comparison are (i) the Rayleigh limit for diffraction limited optics of 1 m and 50 m diameter (black lines), (ii) the angular resolution of current X-ray instruments (blues lines) and of the Hubble and James Webb Space Telescopes (green lines), (iii) the best resolution of some example optical interferometer systems (green triangles) and of some typical radio VLBI measurements (red circles), (iv) the diffraction-limited angular resolution possible with space VLBI (actual and near future; red squares), and (v) dashed lines (cyan) roughly indicating the region in which interstellar scattering becomes dominant at high galactic latitudes (left) and towards the galactic center (right). Note that while continuous lines refer to imaging instruments, the various symbols indicate the finest fringe spacing of interferometers which are not truly imaging.