  Systems with itinerant fermions close to a zero temperature quantum phase
transition like the high temperature superconductors exhibit unusual non-Fermi
liquid properties. The interaction of the long-range and low-energy
fluctuations of the incipient order with the fermions modify the dynamical
properties of the fermions strongly by inducing effective long-range
interactions. Close to the transition the interaction of the order parameter
fluctuations becomes important. In this paper we discuss the physics of the
non-Gaussian order parameter fluctuations on the electronic spectrum and
illustrate their effect by considering the charge-density-wave transition and
the phase fluctuations in a two-dimensional d-wave superconductor.
