For decades, asphalt researchers have been trying to understand and simulate aging behavior of asphalt pavement. After years of related work, the AASHTO R30 protocol was adopted in the mid-1980s to simulate short- and long-term aging of asphalt pavement in the lab. Since then, many works have been published and have indicated that this protocol is not sufficient. This paper summarizes the key research on the subject, highlighting methods employed in aging, measures used to examine aging effects, major conclusions, and new knowledge created. In addition, analysis of research gaps from the published data is also presented.