Ordinary nuclear matter, when sufficiently heated or compressed, is expected to turn into a quark-gluon plasma in which the fundamental degrees of freedom ale the quarks that compose neutrons and protons, and, at finite temperature, antiquarks and gluons as well. These lectures begin with an introduction to the elementary physics of quark-gluon plasmas, and a Grief review of our current understanding of the phase transition from ordinary matter to a quark-gluon plasma. The main thrust of these lectures is to describe the microscopic properties of the plasma, including its elementary modes and transport properties, Even in the weak-coupling limit, calculations of the properties of interacting quark-gluon plasmas are beset by infrared divergences associated with the fact that magnetic interactions, i.e., those occurring through exchange of transverse gluons, are, in the absence of a ''magnetic mass'' in qcd, not screened. We discuss the effects of magnetic interactions on the transport coefficients and the quasiparticle structure of quark-gluon plasmas, describing how inclusion of dynamical screening effects - corresponding to Landau damping of the virtual quanta exchanged - leads to finite transport scattering rates, We illustrate the breakdown of the quasiparticle structure of degenerate plasmas caused by long-ranged magnetic interactions, describe the structure of fermion quasiparticles in hot relativistic plasmas, and touch briefly on the problem of the lifetime of quasiparticles in the presence of long-ranged magnetic interactions.
