Statistical Energy Analysis (SEA) is a method for estimating the approximate dynamic response of coupled, mechanical or acoustical, elements at high frequencies. The underlying theory behind SEA is fundamentally different from that in deterministic methods such as the finite element method, and so is the way the structure is modelled. ESDU 99009 gives an introduction to the reasoning behind SEA, its fundamental theory, its range of application, and its limitations. The parameters and concepts employed in SEA are introduced and explained: energy, modal density, modal overlap, loss factors, coupling loss factors, subsystems, spatial averaging, ensemble averaging, weak/strong coupling, etc. The general step-by-step procedure for creating an SEA model for a system is outlined. Once that SEA model has been established and the SEA parameters have been determined, the spatially-averaged velocities and sound pressure levels for the various elements can be found by solving a simple matrix equation. This procedure of creating an SEA model is illustrated by considering a simple plate/beam structure and the SEA results are compared with those from a finite element calculation. The difficulties of determining the SEA parameters, variance of the results, etc. and the conditions under which the SEA is likely to yield good or poor results are discussed. An extensive number of references is given to fundamental books and publications on SEA, both the theory and its application.