This book presents the latest developments and issues in both experimental and theoretical studies of multi-photon processes and the spectroscopy of atoms, ions and molecules in physics, chemistry, biology and material science. It contains review papers suitable for both active researchers and non-experts who wish to enter the field.Special attention is paid to the recent progress of non-linear photon-matter interactions in atoms, molecules and interfaces: XUV/soft X-ray, high-order harmonic generation in attosecond regime, high-order harmonic generation, sum frequency generation, four-wave mixing spectroscopy and molecular orientation with combined electrostatic and intense, non-resonant laser fields.