Binary star astrometry with milli and sub-milli arcsecond precision

The past several decades have seen accelerating progress in improving binary stars fundamental parameters determinations, as new observational techniques have produced visual orbits of many spectroscopic binaries with a milli arcsecond precision. Modern astrometry is rapidly approaching the goal of sub-milli arcsecond precision, and although presently this precision has been achieved only for a limited number of binary stars, in the near future this will become a standard for very large number of objects. In this paper we review the representative results of techniques which have already allowed the sub-milli arcsecond precision like the optical long baseline interferometry, as well as the precursor techniques such as speckle interferometry, adaptive optics and aperture masking. These techniques provide a step forward from milli to sub-milli arcsecond precision, allowing even short period binaries to be resolved, and often resulting in orbits allowing precisions in stellar dynamical masses better than 1%. We point out that such unprecedented precisions should allow for a significant improvement of our comprehension of stellar physics and other related astrophysical topics.