--------------------- Why Feynman diagrams? --------------------- Feynman diagrams resemble processes with particles moving in space and time, and are often figurately treated as such. But in fact they do _not_ describe such processes, but certain multiple integrals. (To emphasize this, the particles involved in Feynman diagrams are called 'virtual particles' - except for the lines sticking out; these are real particles to be prepared or observed. (Still, many people think mistakenly that virtual particles are somehow also real. See the entries about virtual particles elsewhere in this FAQ.) Although it is nowhere said explicitly, Feynman diagrams are just a mnemonic for nicely picturing the composition of higher order tensors. Create for each tensor of a theory a different vertex type, draw a vertex of this type for each occurence of this tensor in a product expression in Einstein summation convention, and draw a line between two such vertices whenever they share an index to be summed over. The form of the lines defines the value of the coefficient function in such a product, and the sum over Feynman diagrams simply means that one considers a linear combination of these products, integrated over the arguments. Thus this defines a generic representation of an expansion of a function of the tensors of the theory. Tuus Feynman diagrams can be used whenever one expands a function of one or more tensors into a linear combination of products of components of these tensors. Indeed, for this reason, they are also used in classical statistical mechanics and in the analysis of stochastic differential equations by functional integration techniques.