This collection of recent results reports transport-property measurements on high-Tc superconductors and investigations by means of microwave conductivity, optical spectroscopy, high-energy spectroscopy, and nuclear magnetic resonance. Further contributions consider problems related to structure, doping, the electron-phonon interaction, electron correlation, and mechanisms of high-Tc superconductivity. Reviews providing introductions for nonspecialists to particular areas are followed by research reports detailing our current understanding.