In Quantum Field Theory the event of non-renormalizability appears frequently. Examples: non-linear sigma model, general relativity, field redefinition, massive Yang-Mills. I have done some work in few cases. In this seminar I present the general idea, the techniques, the price and the results. The non-linear sigma model will be the guide and the example. I conclude by giving a survey of the results obtained and of the problems encountered for other more complex theories.