  GRANDMA is a world-wide collaboration with the primary scientific goal of
studying gravitational-wave sources, discovering their electromagnetic
counterparts and characterizing their emission. GRANDMA involves astronomers,
astrophysicists, gravitational-wave physicists, and theorists. GRANDMA is now a
truly global network of telescopes, with (so far) 30 telescopes in both
hemispheres. It incorporates a citizen science programme (Kilonova-Catcher)
which constitutes an opportunity to spread the interest in time-domain
astronomy. The telescope network is an heterogeneous set of already-existing
observing facilities that operate coordinated as a single observatory. Within
the network there are wide-field imagers that can observe large areas of the
sky to search for optical counterparts, narrow-field instruments that do
targeted searches within a predefined list of host-galaxy candidates, and
larger telescopes that are devoted to characterization and follow-up of the
identified counterparts. Here we present an overview of GRANDMA after the third
observing run of the LIGO/VIRGO gravitational-wave observatories in $2019-2020$
and its ongoing preparation for the forthcoming fourth observational campaign
(O4). Additionally, we review the potential of GRANDMA for the discovery and
follow-up of other types of astronomical transients.
