  PLATO (PLAnetary Transits and Oscillations of stars) is ESA's M3 mission
designed to detect and characterise extrasolar planets and perform
asteroseismic monitoring of a large number of stars. PLATO will detect small
planets (down to <2 R_(Earth)) around bright stars (<11 mag), including
terrestrial planets in the habitable zone of solar-like stars. With the
complement of radial velocity observations from the ground, planets will be
characterised for their radius, mass, and age with high accuracy (5 %, 10 %, 10
% for an Earth-Sun combination respectively). PLATO will provide us with a
large-scale catalogue of well-characterised small planets up to intermediate
orbital periods, relevant for a meaningful comparison to planet formation
theories and to better understand planet evolution. It will make possible
comparative exoplanetology to place our Solar System planets in a broader
context. In parallel, PLATO will study (host) stars using asteroseismology,
allowing us to determine the stellar properties with high accuracy,
substantially enhancing our knowledge of stellar structure and evolution.
  The payload instrument consists of 26 cameras with 12cm aperture each. For at
least four years, the mission will perform high-precision photometric
measurements. Here we review the science objectives, present PLATO's target
samples and fields, provide an overview of expected core science performance as
well as a description of the instrument and the mission profile at the
beginning of the serial production of the flight cameras. PLATO is scheduled
for a launch date end 2026. This overview therefore provides a summary of the
mission to the community in preparation of the upcoming operational phases.
