Regional security complex theory (RSCT) is a theory of regional security advanced by Barry Buzan and Ole Waever in their 2003 work Regions and Powers: The Structure of International Security. The concept of regional security complexes covers how security is clustered in geographically shaped regions. Security concerns do not travel well over distances and threats are therefore most likely to occur in the region. The security of each actor in a region interacts with the security of the other actors. There is often intense security interdependence within a region, but not between regions, which is what defines a region and what makes regional security an interesting area of study. Buffer states sometimes isolate regions, such as Afghanistan's location between the Middle East and South Asia.