  In this article we focus our attention on the principle of energy
conservation within the context of systems of fluid dynamics. We give an
overview of results concerning the resolution of the famous Onsager conjecture
- which states regularity requirements for weak solutions of the incompressible
Euler system to conserve energy. Further we survey results providing optimal
sufficient regularity conditions for energy conservation for other balance
laws: compressible Euler, Navier-Stokes, magnetohydrodynamics and general
conservation laws.
