The foundation of Roman law was deeply rooted in the customs and traditions of early Roman society. Before formal legal codification, Roman law was based on unwritten customs passed down through generations. These customs, known as mos maiorum or "the way of the ancestors," guided legal practices and social norms. However, as Rome expanded, the need for a more structured and consistent legal system became evident. This led to the development of one of the earliest recorded legal codes, the Twelve Tables, which became the cornerstone of Roman law.

The Twelve Tables, established around 450 BCE, were a response to demands from the plebeians for greater legal transparency. Prior to this, laws were primarily controlled by the patrician class, leaving the lower classes vulnerable to arbitrary decisions. The Twelve Tables provided a written and publicly displayed set of laws that covered various aspects of daily life, including property rights, contracts, family matters, and criminal offenses. While these laws were often harsh by modern standards, they were crucial in establishing the principle that law should be known and applied equally to all citizens.

Legal interpretation played a crucial role in the early Roman legal system, and the pontiffs, a group of religious and legal authorities, were responsible for explaining and applying the laws. Since early Roman law was closely linked to religious practices, the pontiffs held significant influence over legal matters. Over time, however, legal decision-making shifted from religious authorities to magistrates and elected officials, paving the way for a more secular and systematic approach to governance.