There are two basic kinds of norms in every modern legal system: rules and principles. They are applied by means of two different rational procedures: subsumption and balancing. While rules apply by means of subsumption, balancing is the way to apply principles. For this reason, balancing has become an essential methodological criterion for adjudication, especially for the adjudication of fundamental rights, which have the structure of principles. However, balancing is at the heart of many theoretical discussions. One of the most important questions is whether balancing has a rational structure, and whether balancing is a rational procedure or a mere rhetoric device, useful to justify any kind of judicial decisions. In A Theory of Constitutional Rights and in other papers, Alexy supports the thesis that balancing has a rational structure and offers a well developed conception of the structure of balancing. In the last version, three elements form the structure of balancing: the law of balancing, the weight formula, and the burden of argumentation. The aim of this chapter is to analyze the role and the structure of the second element: the weight formula (III), but first it is necessary to clarify the concept and the general structure of balancing (II).