The use of the term “principle” is common to law, but it has a wide variety of different meanings. Contemporary legal theory has added some new ones which should help improve the application of justice, particularly when they are accompanied by an appropriate theory for the interpretation of rules. This article revisits the term “principle” and analyzes the distinction between principles and rules in order to evaluate the different ways in which Mexican Courts apply principles. “Balancing” and “subsumption” are compared as methods when there are conflicts between norms.