  The Planet Formation Imager (PFI) project aims to provide a strong scientific
vision for ground-based optical astronomy beyond the upcoming generation of
Extremely Large Telescopes. We make the case that a breakthrough in angular
resolution imaging capabilities is required in order to unravel the processes
involved in planet formation. PFI will be optimised to provide a complete
census of the protoplanet population at all stellocentric radii and over the
age range from 0.1 to about 100 Myr. Within this age period, planetary systems
undergo dramatic changes and the final architecture of planetary systems is
determined. Our goal is to study the planetary birth on the natural spatial
scale where the material is assembled, which is the "Hill Sphere" of the
forming planet, and to characterise the protoplanetary cores by measuring their
masses and physical properties. Our science working group has investigated the
observational characteristics of these young protoplanets as well as the
migration mechanisms that might alter the system architecture. We simulated the
imprints that the planets leave in the disk and study how PFI could
revolutionise areas ranging from exoplanet to extragalactic science. In this
contribution we outline the key science drivers of PFI and discuss the
requirements that will guide the technology choices, the site selection, and
potential science/technology tradeoffs.
