The precision of the Gaia survey is sufficient not just to identify precise astrometric properties of a huge number of stars, but also to identify many small but significant discrepancies between the observed motion of a star and any plausible single-star model. The most commonplace reason for this discrepancy is that the source is actually a binary (or higher multiple) system — with an orbit too tight for the point sources to be individually resolvable but large enough to add another component to the motion of the centre of light on-sky. Thus, by looking for stars with poorly converged (single-body) astrometric solutions we can identify a great number of binary systems which were previously indistinguishable from point sources. In particular we are sensitive to systems with periods from a few months to a decade, largely independent of the type of either primary or secondary component. In this talk we present results covering the classification and properties of populations of sources, and for the first time classification of individual multi-star systems from astrometric error.