  We develop a theoretical framework to quantify the structure of unstable
hadron resonances. With the help of the corresponding system in a finite
volume, we define the compositeness of resonance states which can be
interpreted as a probability. This framework is used to study the structure of
the scalar mesons f_0(980) and a_0(980). In both mesons, the Kbar K component
dominates about a half of the wave function. The method is also applied to the
Lambda(1405) resonance. We argue that a single energy level in finite volume
represents the two eigenstates in infinite volume. The KbarK N component of
Lambda(1405), including contributions from both eigenstates, is found to be
58%, and the rest is composed of the pi Sigma and other channels.
