  We review the class of materials known as polar metals, in which polarity and
metallicity coexist in the same phase. While the notion of polar metals was
first invoked more than 50 years ago, their practical realization has proved
challenging, since the itinerant carriers required for metallicity tend to
screen any polarization. Huge progress has been made in the last decade, with
many mechanisms for combining polarity and metallicity proposed, and the first
examples, LiOsO3 and WTe2, identified experimentally. The availability of polar
metallic samples has opened a new paradigm in polar metal research, with
implications in the fields of topology, ferroelectricity, magnetoelectricity,
spintronics, and superconductivity. Here, we review the principles and
techniques that have been developed to design and engineer polar metals and
describe some of their interesting properties, with a focus on the most
promising directions for future work.
