Abstract: Less than a decade after the beginning of the Diocletianic Persecution (303 CE), the emperor Constantine changed the course of religious history when, on the eve of battle in 312 CE, he perceived a symbol in the sky. He would later attribute this divine vision as a sign of support from the God of the Christians, convert to Christianity, and employ the symbol – the ‘Constantinian’ monogram – on his new standard, the labarum. However, despite this seemingly straightforward narrative, there has been much discussion surrounding this pivotal event, and several points of contention have continued to arise amongst scholars. For instance, the primary sources appear to describe two different symbols (the chi-rho and the tau-rho), which has led to some confusion about the exact form the ‘Constantinian’ monogram took. In addition, the secondary scholarship tends to be split into two groups, either emphasizing the Christian signification of the symbol or its link to the Constantinian dynasty. More recently, however, scholars have focused on the multivalent meanings of the symbol, thus highlighting its appeal to various groups within the Roman Empire. With a focus on social semiotic interpretation (the intersection between sign-user, context, and cultural background), my dissertation examines the meaning of both the chi-rho and the tau-rho as fourth-century symbols employed in Roman art. The core of data comes from coins and inscriptions, which can be dated and (usually) situated within a specific provenance. These examples have been further supplemented by an analysis of the pre-Constantinian uses of the symbols, the ancient sources (Eusebius, Lactantius, and Optatian), and other artefacts that have been dated to the fourth century with less certainty. Although the development of the chi-rho as a symbol throughout the fourth century is highly complex, my study has found that when it first began to be used by Constantine, it was deployed mainly as a militaristic and dynastic symbol, a testament to the emperor’s protection under God. By the middle of the fourth century, it was used more and more frequently in Christian contexts. However, I contend that above all else, the chi-rho was a powerful protective device, one that continues to be used by the Church today. Before the fourth century, the tau-rho was already in use as a Christian scribal device and was plausibly the first visual representation of the crucifixion of Jesus, thereby firmly establishing it within a Christian milieu early on. Although not a military symbol like the chi-rho, compelling evidence suggests the tau-rho became a dynastic symbol for the Theodosian dynasty towards the end of the fourth century, which parallels the adoption of the chi-rho by the Constantinians. Lastly, this dissertation demonstrates that the chi-rho and the tau-rho held separate meanings, and the tau-rho was not a later derivative of the chi-rho. These results hold far-reaching implications for the future study of ‘Christian’ art in the fourth century and open other avenues (such as expanding the scope of the geographical and chronological evidence) to pursue.