Abstract: In the process of urban renewal and the transformation of transportation infrastructure, a substantial number of old piles are retained following the demolition and renovation of existing structures. The reuse of the existing (old) piles is a crucial pillar in advancing China's strategy for low-carbon urban renewal, which can effectively reduce construction waste and carbon emissions, and save construction cost. In this study, a systematic review is presented on the current research status and classification methodologies for the reuse of existing piles in urban renewal processes. The research on the interaction mechanisms and bearing performance between new and existing piles is introduced. The three fundamental evaluation criteria, namely integrity, durability, and bearing capacity, and their detailed detection methods for existing piles are mainly discussed here. Moreover, the current reinforcement and transformation technologies of the existing piles are summarized in this work. The practical application effect of the existing piles is evaluated based on the representative engineering cases worldwide and revealed that it has significant advantages in construction period compression, carbon emission reduction, and economic and environmental protection. However, research on the interaction mechanisms between new and existing piles, as well as on the standardized assessment frameworks and intelligent inspection methods for existing piles, needs to be further investigated. Finally, some suggestions and prospects about the future study on the reuse of existing piles are put forward.