Nucleon resonances are a direct consequence of the quark/gluon substructure of baryons and are being currently studied with a wide variety of tools, e.g. lattice QCD, quark models, large N_c QCD. The main content of the seminar will deal with the extraction and interpretation of nucleon resonance properties from the experimental data of meson production reactions. The approach followed at the Excited Baryon Analysis Center (EBAC) at Jefferson Laboratory will be presented in detail [1-3]. A recent important development on the nature of P11 nucleon resonances will also be discussed [4]. [1] B. Julia-Diaz, T.-S.H. Lee, A. Matsuyama, and T. Sato, PRC 76, 065201 (2007) [2] B. Julia-Diaz, T.-S.H. Lee, A. Matsuyama, T. Sato and L.C. Smith, PRC 77, 045205 (2008) [3] B. Julia-Diaz, H. Kamano, T.-S.H Lee, A. Matsuyama, T. Sato, N. Suzuki, PRC80, 025207 (2009)[4] N. Suzuki, B. Julia-Diaz, H. Kamano, T.-S.H. Lee, A. Matsuyama, T. Sato, PRL104, 042302 (2010).