LIGO and Virgo have inaugurated the era of Gravitational
Wave Astronomy by observing several binary black hole mergers
and the merger of a neutron star binary.  They have also made
possible multimessenger astronomy with gravitational waves producing
key discoveries.  At the same time there are efforts to detect
gravitational waves beyond the high-frequency band of the ground-based
detectors.  In particular, the European Space Agency has approved
LISA (Laser Interferometer Space Antenna) as its third large-classe
mission.  The main goal of LISA is to carry out the science of the
white paper “The Gravitational Universe”, consisting in low-frequency
gravitational-wave astronomy.  In this talk I will describe the
main scientific goals of the mission and its potential for new
discoveries in astrophysics, cosmology, and fundamental physics.