(defended on dec. 13 1996, at LAPTH, Annecy-Le-Vieux, France)
The theory of weak interactions contains an important question: why are the gauge bosons (W+ W- and Z) massive particles? Several models exist that may explain this experimental fact, and these models may also have some experimental consequences. We thus have studied the implications of some models beyond the Standard Model in the Z ==> gamma gamma gamma decay. The simplest model explaining the W and the Z mass contains a scalar particle called the Higgs boson, and some future colliders will try to discover this particle. However, this won't be sufficient to understand the mechanism that makes the W and the Z massive. Among the interactions needed to realize this spontaneous symmetry breakdown, there exist some interactions between several Higgs bosons. It is therefore needed to measure the corresponding couplings. We have shown that the triple-Higgs interaction is reasonably measurable in the next e+,e- linear collider (in the TeV range). In order to get this result, we had to complete a thorough phenomenological study of some decay processes. gamma gamma ==> W+ W^- H H and e+ e- ==> nu_e anti-nu_e H H were the most interesting processes. For the purpose of this study, we used some new gauges derived from some background field gauges that were shown to be quite interesting. The use of some structure functions was shown to be a powerful way to reproduce the exact computations, and pointed out the dominance of longitudinal W fusion in the latter processes. We tried to improve the structure function formalism, and we have shown the limitations of validity for this formalism.