This paper investigates the Spanish concessive marker aunque from a grammaticalization perspective. Prior studies on aunque focused on the alternation between the indicative and subjunctive moods in aunque clauses. Generally, the indicative mood expresses objective and factual information, while the subjunctive mood expresses subjective and hypothetical information. However, this dichotomy leaves out a number of exceptional examples that cannot be explained. This paper shows how the various uses of aunque are organically interwoven. Diachronically, aun was divided into concessive aun and temporal aún, and attached to concessive que, forming concessive aunque. In its early stage, it was used only for hypothetical contexts, competing with the other concessive marker maguer que. Over time, aunque has expanded to factual contexts with the indicative. Nowadays, it is the most representative concessive marker in the Spanish language. In this process, a cognitive meaning chain is observed: “continuation → inclusion → coexistence → confrontation → unexpectedness → concession.” Also, a branch chain of “confrontation → complement → excuses → refutation → dismissal” is observed when the subjective evaluation of the enunciator is presented through a fact described with the subjunctive mood.