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The present stage of development of long-baseline interferometry using independent atomic frequency standards is reviewed. The technique is being applied to precision geodesy and astrometry and shows great promise as a means for the intercomparison of national time scales at the 1-ns error level. This paper discusses the development of radio interferometry and, in particular, the theory and practice of interferometry over a very wide (effective) bandwidth. The superb phase stability of the hydrogen maser frequency standard in the range of 100 s to several hours makes it the ideal local oscillator for long-baseline Interferometry.