Excursions in QCD.

We study various aspects of QCD using both perturbative and non-perturbative techniques. In the first part of this work, we study QCD in extreme conditions. We examine the effect of finite size on the thermodynamic properties of quark gluon plasma at high temperature. The effects are found to be small if the typical size is larger than 1 fm. We study the fate of Charmonium particle in a dynamically confined plasma. The decay width of the Charmonium due to thermal gluons was found to be around 10 MeV in a quark gluon plasma at temperature of around 200 MeV. As an attempt to study the instability of the perturbative vacuum, we perform a lattice gauge theory calculation of the energy of gauge fields in a box with twisted boundary conditions. The energy of the gauge fields without magnetic flux was found to be lower than that with a unit magnetic flux, which is contrary to a previous speculation. In the second half of this work, we study phenomenological aspects of QCD in the Skyrme-model. We revisit the problem of quantization of the Skyrme-model with vector mesons including the fluctuating pion. Various baryon-meson couplings are obtained, consistent with the soft-pion theorems, and new insight to the 1/N_{c} correction to physical observables are given.