Masers as probes of the gas dynamics close to forming high-mass stars

Imaging the inner few 1000 AU around massive forming stars, at typical distances of several kpc, requires angular resolutions of better than 0''.1. Very Long Baseline Interferometry (VLBI) observations of interstellar molecular masers probe scales as small as a few AU, whereas (new-generation) centimeter and millimeter interferometers allow us to map scales of the order of a few 100 AU. Combining these informations all together, it presently provides the most powerful technique to trace the complex gas motions in the proto-stellar environment. In this work, we review a few compelling examples of this technique and summarize our findings.