Reasoning is an important cognitive activity in probability assessment, and one that has been understudied. This proposition motivates the paper's three general aims. First, based on research in rhetoric, we present a theoretical means of analyzing the arguments constructed during the reasoning that occurs in probability assessment. Second, from verbal protocol data, we establish that subjects constructed arguments in forming beliefs and assessing the associated probabilities. Third, we analyze the data for the structure of subjects' arguments, including argument content and form. Subjects used a limited amount of relevant evidence and used a variety of argument forms that could be characterized by the nature of the knowledge that subjects brought to bear in forming the arguments. Subjects predominantly used causal reasoning, but also employed hierarchical category knowledge, resemblance relationships, and arguments from authority. These findings form a basis for expanding our accounts of probability assessment and for improving assessment practice.