Abstract: Using multi-epoch water maser VLBI observations it is possible to monitor the detailed shapes and 3D kinematics of protostellar jet bowshocks at AU scales, defining the boundaries between the outflowing jet material and the ambient envelope
medium. Tracking the 3D kinematics of these maser bowshocks reveal the jet's physical properties, which can in turn be used to infer the nature of the driving source even when the star itself is undetected due to obscuration. In this colloquium I'll demonstrate the use of maser bowshock observations in protostellar jets as a diagnostic tool for investigating low- and high-mass star formation.