Traditionally, legal systems have been classified as either Common Law or Civil Law; scholars distinguish these systems based on their origins, as well their attitudes towards stare decisis. Common law considers precedent as a source of binding rules, while civil law does not. However, some scholars consider the methods for legal reasoning to be almost the same in every legal system. These scholars maintain that regardless of the source of law in a particular country, once a judge determines that the facts of one case are similar to those regulated by a certain rule, the judge will apply that particular rule and resolve the dispute accordingly. With these similarities and differences of different legal systems in mind, the purpose of this paper is to give an overview of the role of precedent as a source of law in the American legal system. This paper also compares the legal systems of Mexico, as a civil law country, and the United States, as a common law country, to show that the differences between these legal systems result primarily from the preeminence of precedent as a source of law in the US.