21st Century Astrometry and its Science Applications

Dramatic advances in astronomical instrumentation and data processing power have transformed astrometry, over the last decades, from a narrow specialist field into a powerful standard tool for astronomical research. In the optical regime Gaia is the prime example of this, but parallel developments are happening in groundbased techniques and at other wavelengths. In this talk I outline some of the advances that have enabled this transformation and give examples from the amazingly rich variety of science applications of modern astrometry. Many more examples are given elsewhere at this meeting. It is clear that we have only seen the beginning of thís process: Gaia has yet to demonstrate the full extent of its capabilities and will be much expanded in depth and scope by other projects, the full synergies of which may only become apparent much later. The review ends with a few reflections on the long-term prospects and challenges of astrometry.