Lectures on Perturbative QCD at high densities

A two-hour lecture on the basics of in-medium field theory as applied to cold quark matter at high densities. Given as part of a course on Neutron Stars offerered at the Galician Institute of High Energy Physics (IGFAE) in 2021. Topics covered: (i) framework of relativistic thermal and high‑density perturbation theory, (ii) infrared problems in thermal field theory and their resolution for the pressure, (iii) the general structure of the perturbative QCD pressure, and (iv) current status of theoretical calculations.