No astronomical body is alone - some fraction of the mass of a system will always be found in orbiting bodies, ranging from fragmented debris to planets, stars and compact objects. In general we ‘see’ the brighter component: one star often dominates the detectable light, obscuring the companion - and we must turn to indirect methods to infer its presence. Astrometry, the mapping of the motion of stars across the sky, is one such method. A significant mass concentrated in one or more collapsed companions will perturb the path. In the simplest and most ubiquitous case, a binary system, this perturbation is regular and easily described - a metronome beat added to the celestial motion. However, the combination of a star’s proper and parallactic motion with a binary orbit is non-linear, biasing our measurements of the system’s distance and motion and introducing extra astrometric noise. In this thesis we will detail the astrometric impact of a binary companion, how it changes our inferences about systems assumed to be single bodies, and how we can detect companions through the deviations from single body fits. This is especially relevant to the current and ongoing Gaia survey, which is building astrometric tracks at unprecedented (sub milli-arcsecond) precision and for the first time allowing us to detect a large number of unresolved but astrometrically significant binary systems. We present analytic expressions for the binary contribution, as well as an exploration of the behavior of simulated systems and detections of real binary candidates in the local galaxy.