  Recent studies on meson condensations (pions and kaons) in high-density
hadronic matter are reviewed. After summarizing onset mechanisms of pion and
kaon condensations, we discuss implications for neutron star phenomena such as
rapid cooling through neutrino emission, static and dynamic properties of
neutron stars. Recent studies on coexistence problem of pion and kaon
condensations are briefly introduced. Finally, relevance of meson condensations
in hadronic matter to those in color superconductivity are briefly mentioned.
