In this chapter, we explore aspects of quantum field theory related to the topics of lattice field theory, field theory at finite temperature and strong fields. The connection between the last two subjects is that they depart from quantum field theory in the vacuum (e.g., scattering amplitudes for high-energy physics) by considering situations with a large density of particles, in which the quantum field theory formalism needs to be supplemented with aspects of many-body physics. The reason for the inclusion of lattice field theory in this chapter is that, being a Euclidean formulation of QFT, it has a natural finite-temperature interpretation if one views the inverse of the extent of the time direction as a temperature.