  I give a physical explanation of what shear viscosity is, and what physics
determines its value. Then I explain why determining the shear viscosity of the
Quark-Gluon Plasma is interesting. I outline the leading-order calculation of
the QGP shear viscosity (and baryon number diffusion constant), explaining why
the quite complicated physics of parton splitting and Landau-Pomeranchuk-Migdal
interference effects are required for its calculation. Then I briefly explore
the range of applicability, emphasizing the importance of plasma instabilities.
