  We describe an analytical theory investigating the regime of validity of the
Fermi liquid theory in interacting, via the long-range Coulomb coupling,
two-dimensional Fermi systems comparing it with with the corresponding 3D
systems. We find that the 2D Fermi liquid theory and 2D quasiparticles are
robust up to high energies and temperatures of the order of Fermi energy above
the Fermi surface, very similar to the corresponding three-dimensional
situation. We calculate the phase diagram in the frequency-temperature space
separating the collisionless ballistic regime and the collision-dominated
hydrodynamic regime for 2D and 3D interacting electron systems. We also provide
the temperature corrections up to third order for the renormalized effective
mass, and comment on the validity of 2D Wiedemann-Franz law and 2D
Kadawoki-Woods relation.
