This third article in this series (Part III) aims to present new clinical results and long-term follow-up of resin composite inlays and onlays using the modern clinical concepts presented in the Part I and Part II articles. These revised protocols have contributed to eliminating the most frequent difficulties related to the preparation, isolation, impression taking, and cementation of tooth-colored inlays and onlays. This clinical report presents a series of 25 cases of indirect or semidirect inlays and onlays (intra- and extraoral techniques) made of microhybrid and nanohybrid composites with 6- to 21-year follow-ups. The restoration performance was assessed through clinical examination, intraoral radiographs, and clinical photographs. The overall clinical assessment aimed to confirm the absence (success) or presence (failure) of decay or restoration fracture, while the restoration quality was judged on intraoral photographs. The restoration status with regard to margins, anatomy, and color was assessed using three quality scores (A = ideal, B = satisfactory, C = insufficient). Descriptive statistics were used to evaluate the possible impact of composite structure (microhybrid or nanohybrid) or observation time on restoration quality. Over this medium- to long-term observation period, no clinical failure was reported. Only a few restorations (mainly those made of conventional inhomogeneous nanohybrid) presented discrete marginal discoloration (n = 4) or occlusal anatomy change due to wear (n = 7). This first clinical survey with long-term follow-up supports the application of the aforementioned clinical concepts, which thus far have only been validated by in vitro studies.