Hadron Structure and the QCD Phase Transition

Firstly, I give a brief summary of the current understanding of QCD below and near $T_c$ (the critical temperature of the chiral transition). Some emphases are put on the qualitative difference between the Yukawa regime ($T \sim 0)$ and the Hagedorn regime ($T \sim T_c$). Secondly, the dynamical phenomena associated with the chiral transition, in particular, the spectral changes of hadrons in hot and/or dense medium are reviewed from the point of view of the QCD spectral sum rules. Confusions on the QCD sum rules at finite temperature/density are also clarified and remarks on the effective lagrangian approaches are given. Thirdly, planned experiments to detect the spectral changes in medium are summarized.