  The hot circum-galactic medium (CGM) represents the hot gas distributed
beyond the stellar content of the galaxies while typically within their dark
matter halos. It serves as a depository of energy and metal-enriched materials
from galactic feedback and a reservoir from which the galaxy acquires fuels to
form stars. It thus plays a critical role in the coevolution of galaxies and
their environments. X-rays are one of the best ways to trace the hot CGM. I
will briefly review what we have learned about the hot CGM based on X-ray
observations over the past two decades, and what we still do not know. I will
also briefly prospect what may be the foreseeable breakthrough in the next one
or two decades with future X-ray missions.
