Detection and Characterization of Planetary Systems with $\mu$as Astrometry

Astrometry as a technique has so far proved of limited utility when employed as either a follow-up tool or to independently search for planetary mass companions orbiting nearby stars. However, this is bound to change during the next decade. In this review, I start by summarizing past and present efforts to detect planets via milli-arcsecond astrometry. Next, I provide an overview of the variety of technical, statistical, and astrophysical challenges that must be met by future ground-based and space-borne efforts in order to achieve the required degree of astrometric measurement precision. Then, I discuss the planet-finding capabilities of future astrometric observatories aiming at micro-arcsecond precision, with a particular focus on their ability to fully describe multiple-component systems. I conclude by putting astrometry in context, illustrating its potential for important contributions to planetary science, as a complement to other indirect and direct methods for the detection and characterization of planetary systems.