Studies of pion and kaon condensations in high density matter are reviewed from a viewpoint of hadron physics. Mechanisms of these condensations and their implications for astrophysical phenomena such as neutron star cooling, static properties (mass-radius relation ...) of neutron stars, and delayed collapse scenario of hot neutron stars at birth. Effects of nucleon superfluidity on cooling and glitch phenomena are discussed associated with meson condensations. We also would like to discuss the relevance of meson condensations in hadronic matter to those in color superconducting matter toward unified understanding of high density QCD phase diagram.