Since X-ray spectroscopy reaches the core levels, the photon energy of the incident X-rays can be tuned to specific element edges to give element selectivity, and the local electronic structure of a solute can be probed in a complex environment. Hence, XA spectroscopy has been used to study the hydration of amino acids and dipeptides and the influence of salinity on the hydration of proteins. It is a local probe which is primarily determined by covalent bonding, 5 but XA spectra are still strongly influenced by hydration as seen in systematic studies of the pH dependence of nitrogen K-edge XA spectroscopy of aqueous amino acids and alkylamine in protic solvents. Hydrogen-bonding interactions in aqueous ammonia (NH 3 ) and ammonium ion (NH + 4 ), two key amine compounds in aqueous solution, have been the subject of extensive theoretical and experimental investigations in recent years. A quantitative assessment in terms of electronic structure, solvation structure, and dynamics was obtained by combining local soft X-ray and vibrational infrared spectroscopic results with ab initio molecular dynamics simulations based on density functional theory (DFT). The ammonia molecule was shown to have a strong asymmetry in hydrogen bonding to the solvent, with weakly donating hydrogen bonds and a very strong accepting hydrogen bond, whereas the ammonium ion is involved in strong hydrogen bond donation. Theoretical modeling is a fruitful and necessary complement for a trustworthy interpretation of X-ray spectra, but shortcomings of the employed DFT methods for spectrum simulations are apparent. Given improved experimental spectra of solutions, there is a need to develop accurate methods for XA spectrum calculations of explicitly solvated solutes, and it is important to investigate how to best balance the required level of quantum chemistry with the environment. 17