  Reflection positivity (RP) is a property of Gibbs measures exhibited by a
class of lattice spin systems that include the Ising, Potts and Heisenberg
models. The RP property is useful because of its two basic consequences:
infrared bound and chessboard estimates. These are one of basic (and rather
efficient) tools for proving phase transitions in many models of physical
interest. The notes presented hereby summarize the lectures on reflection
positivity and its consequences that the author delivered at the Prague Summer
School on Mathematical Statistical Mechanics in September 2006. The text
features both the classical material on the subject from the late 1970s as well
as some of the more recent developments.
