  Multiply imaged time-variable sources can be used to measure absolute
distances as a function of redshifts and thus determine cosmological
parameters, chiefly the Hubble Constant H$_0$. In the two decades up to 2020,
through a number of observational and conceptual breakthroughs, this so-called
time-delay cosmography has reached a precision sufficient to be an important
independent voice in the current ``Hubble tension'' debate between early- and
late-universe determinations of H$_0$. The 2020s promise to deliver major
advances in time-delay cosmography, owing to the large number of lenses to be
discovered by new and upcoming surveys and the vastly improved capabilities for
follow-up and analysis. In this review -- after a brief summary of the
foundations of the method and recent advances -- we outline the opportunities
for the decade and the challenges that will need to be overcome in order to
meet the goal of the determination of H$_0$ from time-delay cosmography with
1\% precision and accuracy.
