We review the modern view of fluid dynamics as an effective low energy, long wavelength theory of many body systems at finite temperature. We introduce the notion of a nearly perfect fluid, defined by a ratio η/s of shear viscosity to entropy density of order ℏ/k_B or less. Nearly perfect fluids exhibit
hydrodynamic behavior at all distances down to the microscopic length scale of the fluid. We summarize arguments that suggest that there is fundamental limit to fluidity, and review the current experimental situation with regard to measurements of η/s in strongly coupled quantum fluids.
61 pages, prepared for Annual Reviews of Nuclear and Particle Science; Arxive version contains additional endnotes and references