"Path integrals (the terminology for functional integrals, derived from their quantum mechanical origin) had been introduced by Dirac in the 1930s. However, their mathematical fuzziness (in contrast to the precise Euclidean Wiener integrals) had discouraged their serious application in quantum theory. Nonetheless, despite the absence of a mathematical definition it became apparent that path integrals in field theory were ideally suited to (i) implement the symmetries of the theory directly, (ii) incorporate constraints simply, (iii) explore field topology, (iv) isolate relevant dynamical variables, (v) describe non-zero temperature. They were key ingredients in model-making for unified theories, and by the late 1970s a working knowledge of functional integrals had become extremely useful for most field theorists."