  PLATO (PLAnetary Transits and Oscillations of stars) is an ESA M-class
satellite planned for launch by end 2026 and dedicated to the wide-field search
of transiting planets around bright and nearby stars, with a strong focus on
discovering habitable rocky planets hosted by solar-like stars. The choice of
the fields to be pointed at is a crucial task since it has a direct impact on
the scientific return of the mission. In this paper we describe and discuss the
formal requirements and the key scientific prioritization criteria that have to
be taken into account in the Long-duration Observation Phase (LOP) field
selection, and apply a quantitative metric to guide us in this complex
optimization process. We identify two provisional LOP fields, one for each
hemisphere (LOPS1, LOPN1), and discuss their properties and stellar content.
While additional fine-tuning shall be applied to LOP selection before the
definitive choice (to be made two years before launch), we expect their
position will not move by more than a few degrees with respect to what is
proposed in this paper.
