Despite ten years of intensive research, there are still many questions concerning the nature of the electronic structure (Fermi vs non-Fermi liquid) and mechanisms of superconductivity. This book provides an up-to-date and comprehensive review of the experimental results and theoretical interpretations concerning elementary excitation spectra in high-Tc superconductor, both in the normal and superconducting states, and suggests directions where future experimental and theoretical efforts should be concentrated. A critical comparison of different theoretical models involving strong electron correlations, spin fluctuations and electron-phonon coupling is given.