  In this review, we present the most key aspects of modern thermal
perturbation theory based on hard thermal loop (HTL) approximation including
its theoretical foundations and discuss some applications within quantum
electrodynamics (QED) and quantum chromodynamics (QCD) plasma. To maintain
conciseness, our discussion is confined to thermal equilibrium scenarios, but
consider a variety of different quantities and settings. Specifically, we
discuss both bulk thermodynamic properties and real-time observables, spanning
the domain of high temperatures, relevant for heavy-ion physics.
