The emblematic figure of economic thought in the Middle Ages, Thomas Aquinas (c. 1225–1274) is both its most prolific and its best-known contributor. He deals with economic activities in all their diversity and provides, sometimes explicitly and often implicitly, an in-depth analysis of economic risk. Even in the Middle Ages, risk was present in all operations and among all agents. Two main aspects of the elements of understanding of risk provided by Aquinas deserve our attention. Firstly, he highlights a risk that modern formal approaches based on probability distributions have relegated to the background, but which is in fact the risk that determines all the others: the risk of misunderstanding the nature of the activity being handled and the object being exchanged. Secondly, he deploys a multitude of situations involving risks. The diversity of the vocabulary used and the combinations that the classification of these risks makes possible provide analytical tools whose relevance goes beyond the historical framework of the Middle Ages and still speaks to us today.