First systematic study of X-ray emission of liquid samples is presented in this book. Local electronic structures of liquids have been examined. Calculation methodology which is outlined in text has successfully explained the experimental spectra of water (liquids in general) in both gas and liquid phases. It is explained how the strong electron sharing between different water molecules helps to understand the specific spectral features. Possible connection to covalent contribution to the hydrogen bonding is discussed. Special local hydrogen bonding structures in liquid water have been revealed. With the help of calculations, it has been found that molecules in the methanol liquid predominantly persist as hydrogen-bonded clusters. That is new and fresh point of view. Also the applicability of X-ray spectroscopy for determining electronic and chemical structures of liquids is emphasized. It is shown the important role of theoretical simulations in interpreting the experimental measurements. Special attention has been paid to calculations of one powerful X-ray spectroscopy, namely resonant X-ray inelastic scattering (RIXS) spectroscopy.