- The properties of hadrons in dense matter are reviewed. Incorporating these medium effects in the relativistic transport model, which treats consistently the change of hadron masses and energies in dense matter via the scalar and vector fields, heavy-ion collisions at SIS energies are studied. This model is seen to provide satisfactory explanations for the observed enhancement of kaon, antikaon, and antiproton yields as well as the enhanced soft pions in the transverse direction. More direct verifications of changing hadron properties in dense matter through the dilepton invariant mass spectra from heavy-ion collisions are also discussed. 1995.