Before the Norman Conquest in 1066, England was populated by Anglo-Saxton tribes that regulated themselves through customs. When William conquered England in the same year, he proclaimed himself king and declared that all land, and all land-based rights, including the administration of justice, were now vested in the king. To make sure the local courts remained under his control, King William created a group of traveling judges called an Eyre, who represented him and kept an eye on the local courts

This Eyre’s responsibility was to make sure the king received his portion of forfeited property. A secondary responsibility of the Eyre, was to hear common pleas of disputes between the ordinary citizens.

It was the common-plea decisions made by judges in the Eyre that formed the body of legal precedent that became known as the common law. As the judges resolved common-plea disputes, they created precedents to be followed in similar cases, and because the common law was built case by case, it is sometimes called case law. Many of the laws that were created in medieval England became the basis of statutory law in modern England, as well as the United States.

The efforts of the Norman kings to centralize their power over all of England were not very successful and in 1215, the powerful landholders rebelled against the heavy taxation of the King John and eventually made him sign the Magna Carta. The purpose of the Magna Carta was to settle disputes between the king and his nobles by placing checks on royal power. Thus, the Magna Carta has formed the basis of what would later be called due process of law.

In the United States, criminal laws are almost entirely a product of constitutional authority and the legislative bodies that enact them. Criminal laws are products of the lawmaking bodies created by constitutional authority. Federal statutes are enacted by Congress, and state statutes are enacted by state legislature. Laws created by municipalities, by city councils which are called ordinances. Both federal and criminal statutes of each state, the definitions of these crimes, and the penalties associated with them, can be found in the penal codes for each jurisdiction.

Common law, also called case law, is a by-product of decisions made by trial and appellate court judges, who produce case law whenever they render a decision in a particular case. This decision becomes a precedent for deciding the outcomes of similar cases in the future. Even though decisions can be made by any trial judge to become a precedent, it is usually the written decisions of appellate court judges that are required to be in writing, these written decisions become known as common law. The principle of using precedents to guide future decisions in court cases are called stare decisis, which is the Latin term for stand by decided cases.

Although common law was an important source of criminal law in colonial America that is no longer the case today. Today, what were once known as common law crimes, as well as many of the new crimes of today, have been defined by statutes created by legislatures in all states. However, if it were not for the common laws of yesterday, who knows what type of laws we would have today? Common Law still remains an important part of our American history.