In this talk I will present a couple of results for the existence of solutions to the one-dimensional Euler, Navier-Stokes and multi-dimensional Navier-Stokes systems. The purpose of the talk is to focus on the role of the pressure in the compressible fluid equations, and to understand whether or not it can be replaced by the nonlocal attraction-repulsion terms arising in the models of collective behaviour.

We discuss some stability problems related to the Navier-Stokes-Fourier system describing the motion of a compressible, viscous, and heat conducting fluids. We introduce the concept of relative entropy/energy and present some applications that concern:
- Existence and conditional regularity of weak solutions;
- singular limits;
- existence and regularity for the inviscid system.