Clustering in nuclei and hadrons

Clustering phenomena in hadron and nuclear systems are overviewed. Because the energy scales for hadron and nuclear systems are not very much different, we can apply similar ideas and methods for the study of the seemingly different two systems, with the key ingredient, cluster. Once clusters are formed, they develop low lying excitations near the threshold if suitable interaction is available. This is a universal phenomenon which has been known as the threshold-rule. For nuclear systems the most established is the alpha (\(\alpha \)-)cluster which is the nucleus of \(^4\textrm{He}\). How the \(\alpha \)-clusters develop in nuclei of light-medium mass region, typically \(^{12}\textrm{C}\) is discussed in a microscopic manner based on the antisymmetrized molecular dynamics. The evidences of such clustering in some static properties of nuclei and scattering phenomena are discussed. Similar clustering phenomena occur for hadrons, where quark clusters are developed in either colored or colorless channels which are considered to play an important role for exotic hadrons of multi-quarks. In particular colorless hadronic clusters can be good constituents to form molecular-like structure near threshold regions.