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International audience; Commission 8 has regularly published triennial reports in the past and the current OC therefore voted to adopt a traditional format also for this special Legacy issue of the IAU Transactions. The outgoing President is grateful for the support of many Commission members who contributed to this report. Our contribution consists of 3 parts: 1) this introduction, providing a general overview and highlights of recent research in astrometry, 2) a summary of the astrometry business & science meeting at the 2015 IAU General Assembly, and 3) the activity report of our Commisson covering the mid-2012 to mid-2015 period.

The SIM Lite Observatory is expected to provide a global astrometric reference frame surpassing the 1-μas accuracy threshold in some spherical harmonics. A range of time-varying physical distortions of the reference frame will become observable as large-scale perturbations of the proper motion field. I consider the main sources of the apparent and physical motion of reference objects, such as the aberration of light caused by the acceleration of SIM, long gravitational waves and hypothetical rotation of the Universe, and present some estimates of the astrometric sensitivity to these effects. I argue that a global solution and covariance analysis is of crucial importance for the SIM mission to differentiate the inevitable accidental and systematic zonal errors from real physical phenomena.

We simulate the despinning of Mercury, with or without a fluid core, and with a frequency-dependent tidal model employed. The tidal model incorporates the viscoelastic (Maxwell) rebound at low frequencies and a predominantly inelastic (Andrade) creep at higher frequencies. It is combined with a statistically relevant set of histories of Mercury's eccentricity. The tidal model has a dramatic influence on the behaviour of spin histories near spin-orbit resonances. The probabilities of capture into high-order resonances are greatly enhanced. Exploring several scenarios, we conclude that the present 3:2 spin state was achieved by entrapment of an initially prograde cold Mercury when its age was less than 20 Myr, i.e., well before differentiation.

We present a mission overview and science possibilities of space astrometry with the Milli-Arcsecond Pathfinder Survey (MAPS).