  This paper outlines the roadmap towards the redefinition of the second, which
was recently updated by the CCTF Task Force created by the CCTF in 2020. The
main achievements and the open challenges related to the status of the optical
frequency standards, their contribution to time scales and UTC, the possibility
of their comparison and the knowledge of the Earth's gravitational potential at
the necessary level of uncertainty are discussed. In addition, the mandatory
criteria to be achieved before redefinition and their current fulfilment level,
together with the redefinition options based on a single or on a set of
transitions are described.
