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We develop various complementary concepts and techniques for handling quantumfluctuations of Goldstone bosons.We emphasise that one of the consequences ofthe masslessness of Goldstone bosons is that the longitudinal fluctuations alsohave a diverging susceptibility characterised by an anomalous dimension $(d-2)$in space-time dimensions $2<d<4$.In $d=4$ these fluctuations divergelogarithmically in the infrared region.We show the generality of thisphenomenon by providing three arguments based on i). Renormalization groupflows, ii). Ward identities, and iii). Schwinger-Dyson equations.We obtain anexplicit form for the generating functional of one-particle irreduciblevertices of the O(N) (non)--linear $\sigma$--models in the leading 1/Napproximation.We show that this incorporates all infrared behaviour correctlyboth in linear and non-linear $\sigma$-- models. Our techniques provide analternative to chiral perturbation theory.Some consequences are discussedbriefly.Comment: 28 pages,2 Figs, a new section on some universal features of  multipion processes has been adde

INFRARED BEHAVIOR OF SYSTEMS WITH GOLDSTONE BOSONS