The second data release (DR2), which occurred on 25 April 2018, is based on 22 months of observations made between 25 July 2014 and 23 May 2016. It includes positions, parallaxes and proper motions for about 1.3 billion stars and positions of an additional 300 million stars in the magnitude range g = 3–20, red and blue photometric data for about 1.1 billion stars and single colour photometry for an additional 400 million stars, and median radial velocities for about 7 million stars between magnitude 4 and 13. It also contains data for over 14,000 selected Solar System objects. The coordinates in DR2 use the "Gaia" celestial reference frame ("Gaia"–CRF2), which is based on observations of 492,006 sources believed to be quasars and has been described as "the ﬁrst full-ﬂedged optical realisation of the ICRS … built only on extragalactic sources." Comparison of "Gaia"–CRF2 with a preliminary version of the forthcoming ICRF3 shows a global agreement of 20 to 30 μas, although individual sources may differ by several mas. Since the data processing procedure links individual Gaia observations with particular sources on the sky, in some cases the association of observations with sources will be different in the second data release. Consequently, DR2 uses different source identification numbers than DR1. A number of issues have been identified with the DR2 data, including small systematic errors in astrometry and significant contamination of radial velocity values in crowded star fields, which may affect some one percent of the radial velocity values. Ongoing work should resolve these issues in future releases. A guide for researchers using Gaia DR2, which collected "all information, tips and tricks, pitfalls, caveats, and recommendations relevant to" DR2, was prepared by the Gaia Helpdesk in December 2019.