  We discuss Ward identities for strongly interacting fermion systems described
by Eliashberg-type theories. We show that Ward identities are not in conflict
with Migdal theorem. We derive diagrammatically Ward identity for a charge
vertex in a Fermi liquid, and when a
  Fermi liquid is destroyed at quantum criticality. We argue that
  Ward identity for a spin vertex cannot be obtained within
  Eliashberg theory.
