  The Galactic center provides a unique astrophysical laboratory for us to
study various astrophysical processes. In this paper, we review and outline the
latest results from observations of Sgr~A$^*$ in terms of source structure and
variations in flux density. Sgr~A$^*$ phenomenon represents a typical case of
low radiative efficiency accretion flow surrounding a supermassive black hole
in low luminosity AGNs. Many pending astrophysical problems found from
observations of Sgr A$^*$ have challenged the existing astrophysical theories.
Current theoretical models of Sgr A$^*$ are also reviewed.
