  The structure of the hadron resonances attracts much attention, in
association with the recent observations of various exotic hadrons which do not
fit well in the conventional picture. These findings urge us to consider
various new configurations such as the multiquark states and the hadronic
molecules. However, it is a subtle problem to define a proper classification
scheme for the hadron structure, and the nonzero decay width of the hadron
resonances makes the analysis complicated. In this paper, we summarize the
recent developments in the studies of the structure of the hadron resonances,
focusing on the notion of the compositeness in terms of the hadronic degrees of
freedom.
