In the general astronomical community VLBI observations are often considered to be rather exotic, because the technique is regarded as complicated and its range of application limited to AGN. We acknowledge that VLBI observations, and in particular the data calibration, can be a challenge, but we have shown here that the technique is applicable to a surprisingly broad variety of astrophysical objects and problems. We wish to highlight a few key contributions.
The structure and evolution of plasma jets ejected from compact objects such as black holes and neutron stars would not have been possible without VLBI. In particular multi-epoch observations have revealed the evolution of plasma jets and yielded constraints on their structure, physics, and environment.
In the Milky Way a key contribution of VLBI observations are proper motions and parallaxes for stars, masers, and pulsars. These observations are a reference for other measurements of distances and so are fundamental for the interpretation of other observations, and to derive a consistent picture of the Milky Way's structure.
A more general application is the use of VLBI to implement a reference frame, which is used throughout observational astronomy. These observations not only tell us where objects are, but they also tell us how fast the earth rotates, and hence how to set our clocks, and how observations at different wavelengths fit together.
Recent advances in computer technology have had profound impact on VLBI observations, and will do so in the near future. Directly-linked antennas will become the standard mode of operation at many VLBI facilities, and software correlators will allow observers to image wider fields of view, and to use more bandwidth to increase sensitivity and (u, v) coverage. The most relevant improvement however will be the SKA: it will be sensitive enough for observations of thermal sources. At mas-scale resolution, the SKA will image many astronomical objects which are inaccessible today.