The development of techniques to study the liquid-liquid interface is a major challenge. Spectroscopy in all its forms provides a powerful method of investigation, especially when combined with other optical techniques. Over the last 30 years, there have been significant developments in the methods for studying heterogeneous interfaces. As technology progresses, the sensitivity of existing techniques has been improved but there are major challenges still to be met, such as the measurement of interfacial dielectric constant and viscosity. This paper aims to summarise the use of spectroscopy to study molecular interactions at the liquid-liquid interface.