  At its core, hydrodynamics is a many-body low-energy effective theory for the
long-wavelength, long-timescale dynamics of conserved charges in systems close
to thermodynamic equilibrium. It has a wide range of applications spanning from
nuclear physics, astrophysics, cosmology, and more recently
strongly-interacting electronic phases of matter. In solid-state systems,
however, symmetries are often only approximate, and softly broken by the
presence of the lattice, impurities, and defects, or because the symmetry is
accidental. Therefore, the hydrodynamic regime must be expanded to include weak
non-conservation effects, which lead to a theory known as quasihydrodynamics.
In this thesis we make progress in understanding the theory of (quasi)
hydrodynamics, with a specific focus on applications to condensed matter
systems and their holographic description.
