  This review highlights some of the lessons that the holographic gauge/gravity
duality has taught us regarding the behavior of the shear viscosity to entropy
density in strongly coupled field theories. The viscosity to entropy ratio has
been shown to take on a very simple universal value in all gauge theories with
an Einstein gravity dual. Here we describe the origin of this universal ratio,
and focus on how it is modified by generic higher derivative corrections
corresponding to curvature corrections on the gravity side of the duality. In
particular, certain curvature corrections are known to push the viscosity to
entropy ratio below its universal value. This disproves a longstanding
conjecture that such a universal value represents a strict lower bound for any
fluid in nature. We discuss the main developments that have led to insight into
the violation of this bound, and consider whether the consistency of the theory
is responsible for setting a fundamental lower bound on the viscosity to
entropy ratio.
