ALT Sum frequency generation spectroscopy can be utilized to address the challenge of studying solid-liquid interfaces under ambient pressure and with macroscopic amounts of liquid, while still retrieving molecular level information. By utilizing a non-linear optical process, this method allows to selectively measure vibrations of interfacial species. Within this chapter sum frequency generation spectroscopy is first briefly introduced and then comprehensively evaluated as an investigative tool for solid-liquid interfaces, with mineral-water interfaces as model system. Additionally, it is explained how the actual probing depth of this method depends on both the experimental setup and the properties of the investigated system, such as surface charges.