Book excerpt: Clarifying chemical processes in the environment is tantamount to creating a better and a safer planet. The chemistry that takes place within the natural world occurs not only in the bulk gaseous, liquid, and solid phases, but also in the region where two phases meet. This molecularly thin region between phases, also known as an interface, plays a significant role in various chemical processes because interfaces are ubiquitous in nature. Despite the significance of interfacial processes in environmental chemistry, investigating environmental interfaces experimentally has always been a challenge. Recent advances in nonlinear spectroscopy (NLS) have demonstrated that techniques such as sum frequency generation (SFG) and second harmonic generation (SHG) are unique in their ability to probe buried chemical interfaces. The theoretical and practical aspect of these techniques in probing environmental interfaces is the primary focus of this e-book. This e-book is geared toward curious and inquisitive minds eager to learn how molecules behave at the thin layers of chemical interfaces. A beautiful world, rich in unique insights into the interfacial environmental processes, awaits.