After more than a century of explorations at the Sun Pyramid in Teotihuacan, many uncertainties about the function, meaning, chronology, and use of the monument remain unresolved. Here we present preliminary results from excavations and mapping conducted by the Sun Pyramid Project to address some of these issues. We focus particularly on describing the architectural features excavated in the interior of the pyramid as well as the subterranean tunnel. This evidence is used to argue that the subterranean structure was artificially constructed for ritual activities, including the placement of royal burials and/or offerings in its interior. We define three stages in the construction sequence of the Sun Pyramid: (1) a Pre-Sun Pyramid phase, (2) the establishment of the main corpus of the monument, and, finally, (3) the adjoining of the adosada platform. In each phase, we discuss the presence of burials, offerings, or other features uncovered. Furthermore, we present the results of new ceramic and radiocarbon dates that transform our understanding about the construction of the ceremonial center, where the Sun Pyramid and the subterranean tunnel are dated to a later phase than previously thought, from A.D. 170—310 and A.D. 140—240, respectively.