The fundamental theory of strong interaction, quantum chromodynamics, is nonperturbative at low energies. Thus, it is still a challenge to understand the structure of hadrons which are composed of quarks and gluons. Especially, the exotic hadrons beyond the naive quark model have been expected to exist for a long time, but strong evidence only appeared recently due to worldwide experimental efforts including the BESIII experiment in IHEP. These experiments have reported many new particles in the last decade. To understand them, one needs well-devised theoretical methods, among which effective field theory is one of the model-independent approches. In this talk, I will report some insights into the structure of hadrons using effective field theory.