Owing to their compactness and high brightness, molecular masers are ideal targets for high-angular resolution VLBI. At cm wavelengths, intense maser transitions, in particular the 6.7 GHz CH$_3$OH and the 22.2 GHz H$_2$O lines, are extensively observed in high-mass star forming regions and provide unique diagnostic probes. Multi-epoch VLBI observations can provide the 3D kinematics of the circumstellar gas and sensitive polarimetric observations can yield the strength and structure of the magnetic field around forming stars on scales (10-1000 AU) not accessible with other techniques. At mm wavelengths, VLBI imaging of masers is largely unexplored, with the only exception of SiO transitions (at 43 GHz and 86 GHz), which enabled us to exceptionally resolve the launch and collimation region of an outflow from a compact disk in the closest known high-mass protostar. Maser transitions from methanol and water have also been detected at mm and submm wavelengths with single-dish telescopes (in the frequency range 80-700 GHz). Multi-frequency VLBI observations from cm to mm wavelengths would be very useful: I) to infer the physical conditions from maser line ratios (if they originate from the same gas); II) to map out more of the source structure, dynamics, and physical conditions (if they probe different portions of the circumstellar gas); III) to measure proper motions with higher accuracy on shorter time baselines at higher frequencies (owing to smaller synthesised beams).
Prospects of imaging mm masers with VLBI will be discussed (time permitting).