  The Virial Thoerem is a mathematical expression obtained from the equation of
motion for a fluid, which describes the energy budget of particular regions
within the flow. This course reviews the basic theory leading to the Virial
Theorem, discusses its applicability and limitations, and then summarizes
observational results concerning the physical and statistical properties of
interstellar clouds which are normally understood in terms of the Virial
Theorem, in particular the so-called ``Larson's Relations''. In all cases, the
standard notions as well as relevant counterpoint are presented. In particular,
the difficulties arising when the medium is fully turbulent are discussed.
