  We study the consequences of Coulomb interactions on a system undergoing a
putative first order phase transition. In two dimensions (2D), near the
critical density, the system is universally unstable to the formation of new
intermediate phases, which we call ``electronic microemulsion phases,'' which
consist of an intermediate scale mixture of regions of the two competing
phases. A correlary is that there can be no direct transition as a function of
density from a 2D Wigner crystal to a uniform electron liquid. In 3D, %we find
that if the strength of the Coulomb interactions exceeds a critical value, no
phase separation occurs, while for weaker Coulomb strength, electronic
microemulsions are inevitable. This tendency is considerably more pronounced in
anisotropic (quasi 2D or quasi 1D) systems, where a devil's staircase of
transitions is possible.
