Energy Budget and the Virial Theorem in Interstellar Clouds

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.