Cicero’s practice of dialectic has been analyzed in his philosophical or rhetorical treatises, but not in his letters. While the letters sometimes offer very technical developments, they do not constitute a theoretical corpus and the dialectic we find there, which has an Academic, a Peripatetic or a Stoic origin, is often a soft echo of something he has discussed in his rhetorical or philosophical books, either to praise it or to criticize it, particularly in his polemic against the Stoics. In this chapter, I study the heterogeneous, diffuse and sometimes tenuous presence of dialectic, upon which Cicero himself made a balanced judgement, considering it sometimes as a tool (albeit an important one) with which the orator could make his style richer, sometimes as an instrument necessary to the intellectual rigor of philosophical reasoning. In order to define more precisely my field of investigation, I begin with a study of Cicero's vocabulary and practice of dialectic in most of his works and in his letters. Then I analyze the influence of jin utramque partem on the correspondence as well as its dialogical, and sometimes agonistic, nature.