The present study begins with a reference to the principal systems of extension of the prejudice in the compared western Law, to estimate that the problems or questions in all of them are the same, and the answers that have been given do not present big differences, since about the first moment it could be thought. The fundamental part of the work is dedicated to the Spanish Law, with undeniable references to the juridical classifications French and Italian. One dedicates a chapter to each of the criteria contemplated by the civil Spanish Code (predictability, necessary consequence and judicial moderation), where close to his historical formation his dogmatic aspects will be studied, and the conclusions reached up to the moment by the jurisprudence. In the specific case of the criterion necessary consequences, it was considered opportune to realize also an analysis of the causality and the straddle inside her of the rule in study, as a juridical own beginning of one of the functions of the causal link. In addition, they dedicate independent chapters for the analysis of the concepts used by the Spanish norm, since they are the debtor of good faith and the unreliable fraudulent one, and the different doubts that in relation with them have been provoked. The second part and end of the work devotes itself to the Chilean right. The useless repetitions are avoided here in possible, and he devotes himself to the analysis of the particular aspects of the Chilean legislation or the peculiar interpretations that his doctrine and jurisprudence has given, bearing in mind always the dogmatic conclusions to which it has come near in the first part of the study. One has tried to include the principal problems that raise the procedure on extension of the prejudice, especially the real meaning and function of each one the criteria. To fulfill such a aim, we must have parted with deep-rooted beliefs on the quantum resarcitorio and to propose new ways of understanding it, bearing in mind always the most attentive doctrine in his study. The task undoubtedly has been complex and difficult, and though a good guide has been had in these stormy waters, it is possible neither to wait nor aspire to give response or definitive solution to the numerous questions that one raises and that they will continue raising. Many of questioning these go thousand five hundred years waiting for a satisfactory response, for two not saying thousand or any more - already JUSTINIANO was qualifying them like dubitationes antiquae in infinitum-. We hope, it yes, that it is a step, between many others that have preceded us, in the production of a modern doctrine on the determination of the contractual hurt in the Spanish and Chilean right.