  The quasi two-dimensional (quasi-2D) kagome materials AV$_3$Sb$_5$ (A=K, Rb,
Cs) were found to be a prime example of kagome superconductors, a new quantum
platform to investigate the interplay between electron correlation effects,
topology and geometric frustration. In this review, we report recent progress
on the experimental and theoretical studies of AV$_3$Sb$_5$ and provide a broad
picture of this fast-developing field in order to stimulate an expanded search
for unconventional kagome superconductors. We review the electronic properties
of AV$_3$Sb$_5$, the experimental measurements of the charge density wave
state, evidence of time-reversal symmetry breaking, and other potential hidden
symmetry breaking in these materials. A variety of theoretical proposals and
models that address the nature of the time-reversal symmetry breaking are
discussed. Finally, we review the superconducting properties of AV$_3$Sb$_5$,
especially the potential pairing symmetries and the interplay between
superconductivity and the charge density wave state.
