Astrometry is the astronomical discipline concerned with the accurate measurement and study of the (changing) positions of celestial objects.

Astrometry has a long history (Perryman 2012) even before the invention of the telescope.

Since then, advances in the instrumentation have steadily improved the achievable angular accuracy, leading to a number of important discoveries: stellar proper motion (Halley 1717), stellar aberration (Bradley 1727), nutation (Bradley 1748), and trigonometric stellar parallax (Bessel 1838; Henderson 1840; von Struve 1840).

Obtaining accurate parallax measurements from the ground, however, remained extremely challenging owing to the difficulty to control systematic errors and overcome the disturbing effects of the Earth’s atmosphere, and the need to correct the measured relative to absolute parallaxes.

Until the mid-1990s, for instance, the number of stars for which ground-based parallaxes were available was limited to just over 8000 (van Altena et al. 1995, but see Finch & Zacharias 2016).

This situation changed dramatically in 1997 with the Hipparcos satellite of the European Space Agency (ESA), which measured the absolute parallax with milli-arcsecond accuracy of as many as 117 955 objects (ESA 1997).

The Hipparcos data have influenced many areas of astronomy (see the review by Perryman 2009), in particular the structure and evolution of stars and the kinematics of stars and stellar groups.

Even with its limited sample size and observed volume, Hipparcos also made significant advances in our knowledge of the structure and dynamics of our Galaxy, the Milky Way.

The ESA astrometric successor mission, Gaia, is expected to completely transform the field.

The main aim of Gaia is to measure the three-dimensional spatial and the three-dimensional velocity distribution of stars and to determine their astrophysical properties, such as surface gravity and effective temperature, to map and understand the formation, structure, and past and future evolution of our Galaxy (see the review by Bland-Hawthorn & Gerhard 2016).

The Milky Way contains a complex mix of stars (and planets), interstellar gas and dust, and dark matter.

These components are widely distributed in age, reflecting their formation history, and in space, reflecting their birth places and subsequent motions.

Objects in the Milky Way move in a variety of orbits that are determined by the gravitational force generated by the integrated mass of baryons and dark matter, and have complex distributions of chemical-element abundances, reflecting star formation and gas-accretion history.

Understanding all these aspects in one coherent picture is the main aim of Gaia.

Such an understanding is clearly also relevant for studies of the high-redshift Universe because a well-studied template galaxy underpins the analysis of unresolved galaxies.

Gaia needs to sample a large, representative, part of the Galaxy, down to a magnitude limit of at least 20 in the Gaia G band to meet its primary science goals and to reach various (kinematic) tracers in the thin and thick disks, bulge, and halo (Perryman et al. 2001, Table 1).

For the 1000 million stars expected down to this limit, Gaia needs to determine their present-day, three-dimensional spatial structure and their threedimensional space motions to determine their orbits and the underlying Galactic gravitational potential and mass distribution.

The astrometry of Gaia delivers absolute parallaxes and transverse kinematics (see Bailer-Jones 2015 on how to derive distances from parallaxes).

Complementary radial-velocity and photometric information complete the kinematic and astrophysical information for a subset of the target objects, including interstellar extinctions and stellar chemical abundances.

Following the Rømer mission proposal from the early 1990s (see Høg 2008), the Gaia mission was proposed by Lennart Lindegren and Michael Perryman in 1993 (for historical details, see Høg 2014), after which a concept and technology study was conducted.

The resulting science case and mission and spacecraft concept are described in Perryman et al. (2001).

In the early phases, Gaia was spelled as GAIA, for Global Astrometric Interferometer for Astrophysics, but the spelling was later changed because the final design was non-interferometric and based on monolithic mirrors and direct imaging and the final operating principle was actually closer to a large Rømer mission than the original GAIA proposal.

After the selection of Gaia in 2000 as an ESA-only mission, followed by further preparatory studies, the implementation phase started in 2006 with the selection of the prime contractor, EADS Astrium (later renamed Airbus Defence and Space), which was responsible for the development and implementation of the spacecraft and payload.