The optimal patent breadth represents a fundamental key for balancing the trade off between desirable and undesirable effects generated by patents. The present paper addresses the dilemma between a broad and a narrow scope by analysing the infringement by equivalency. Hardly any economic and legal literature has been developed with respect to the two main theories in this field, the U.S. Doctrine of Equivalents and the English Catnic/Improver Test. In particular, the Doctrine of Equivalents and the Catnic/Improver Test, as modified by the Protocol on the interpretation of Article 69 of the European Patent Convention (EPC), are compared within a normative analysis in order to establish the more efficient rule. Both patent systems attempt to balance the inventor s and society s interests. Specifically the Doctrine of Equivalents is quite a flexible theory, which is in favour of a broader scope and thus of a patentee s strong monopolistic position, but with the opportunity of some corrective measures to apply case-by-case. On the contrary, the English system entitles quite a narrow scope according to its tradition, partially extended by the EPC. No definitive answers can be provided with respect to the socially desirable patent scope; further researches are required. Nevertheless, the U.S. Doctrine of Equivalents seems to be more efficient than the Catnic/Improver Test. In fact, the U.S. theory is characterised by high flexibility and a case by case approach, paying particularly attention to giving incentives to technological advance; the empirical data, the different solutions provided in some technological areas and some tendencies towards an international harmonization show the consistency and the equilibrium achieved by such theory. However, the American system ought to be improved, especially on the grounds of legal certainty and predictability, while the English and European ones should accelerate the process of convergence towards the American model without losing the extraordinary experience matured in centuries of tradition.