In this PhD thesis the highly surface-specific technique of vibrational sum-frequency generation (VSFG) spectroscopy is described and applied to several aqueous systems. VSFG allows the specific probing of the vibrational resonances of the molecules embedded in single molecular layers at interfaces. The technique also allows the determination of the orientation of the molecular groups carrying the vibration. A detailed description of the theoretical background of VSFG can be found in chapter 2, while in chapter 3 the experimental implementation of the technique is described. In chapter 4 the technique is used to identify the origin of the VSFG response of the bending vibration of water molecules at the surface of pure water and at charged interfaces. In chapter 5 VSFG is used to study the molecular orientation of urea molecules in water at charged interfaces. In chapter 6 the vibrational response and the molecular orientation of small carboxylate ions at the water/air interface are studied. Chapter 7 discusses the origin of the VSFG signal of small carboxylic acids at the water/air interface, while chapter 8 describes the different freezing behavior of aqueous solutions of small carboxylic acids on ice. Chapter 9 reports on a study of the molecular structure of hyperactive antifreeze proteins adsorbed to the surface of ice. In the final chapter, chapter 10, the molecular structure and surface accumulation dynamics of hyaluronan at the water/air interface are discussed, in dependence of the molecular weight and concentration of hyaluronan, and the properties of the solvent.