  The possibility of reconciliation between seemingly mutually exclusive
properties in one system can not only lead to theoretical breakthroughs but
also potential novel applications. The research on the coexistence of two
purportedly contra-indicated properties, ferroelectricity/polarity and
conductivity, proposed by Anderson and Blount over 50 years ago was recently
revitalized by the discovery of the first unambiguous polar metal LiOsO3 and
further fueled by the demonstration of the first switchable ferroelectric metal
WTe2. In this review, we first discuss the reasons why the coexistence of
ferroelectricity/polarity and conductivity have been deemed incompatible,
followed by a review on the history of ferroelectric/polar metals. Secondly, we
review the important milestones along with the corresponding mechanisms for the
ferroelectric/polar metallic phases in these materials. Thirdly, we summarize
the design approaches for ferroelectric/polar metals. Finally, we discuss the
future prospects and potential applications of ferroelectric/polar metals.
