Gaia relies on the proven principles of ESA's Hipparcos mission to help solve one of the most difficult yet deeply fundamental challenges in modern astronomy: the creation of an extraordinarily precise three-dimensional map of about one billion stars throughout our Galaxy and beyond. This massive stellar census provides the basic observational data to tackle an enormous range of important problems related to the origin, structure, and evolutionary history of our Galaxy. The Gaia mission delivers an astronomical catalogue and data archive of unprecedented scope, accuracy and completeness. The Gaia satellite was launched at the end of 2013, this second data release is based on observations collected between 25 July 2014 and 23 May 2016, spanning a period of 22 months of data collection (or 668 days). This second data release, which represents a major advance with respect to Gaia DR1 in terms of completeness, performance, and richness of the data products， consisting of astrometry, photometry, radial velocities, and information on astrophysical parameters and variability, for sources brighter than magnitude 21. In addition epoch astrometry and photometry are provided for a modest sample of minor planets in the solar system. More information is available at https://www.cosmos.esa.int/web/gaia/dr2.