Interfacial electronic structure is of critical importance to a variety of systems including biological, photovoltaics, and spintronic devices. The ability to measure and quantify this interfacial structure is necessary to engineer these systems in a targeted, informed manner; however, efforts to do so are often stymied by the fact that bulk signal tends to overwhelm interfacial signal. There are two experimental approaches to accessing the interface: (1) interface specific spectroscopic techniques and (2) bulk sensitive measurements on thin film samples, which can be considered "all interface." In this Dissertation, I present efforts and achievements in both of these approaches. In Chapter 2 I describe the underlying principles of sum frequency generation (SFG) spectroscopy, an interface specific technique, and the experimental challenges and requirements to achieve robust SFG spectra in the electronic regime. Chapter 3 details the design of a phase stable interferometer that meets many of these challenges and shows a dramatic improvement in the signal-to-noise of SFG spectra. To address the case of thin film samples, I focus in particular on molybdenum disulfide (MoS2) a two-dimensional material that is a direct bandgap semiconductor in the monolayer limit. MoS2 and other transition metal dichalcogenides (TMDs) are of significant interest in modern electronics, due in particular to their tunability, ease of functionalization, and spin split band structure. This system is measured with transient absorption spectroscopy, a bulk sensitive technique, and variants thereof. Chapter 4 describes the spectroscopic and structural properties of MoS2 and other TMDs. Chapter 5 details a particular instance of laser induced modification in monolayer MoS2. Chapter 6 explores the spectral effect of modifying MoS2 with a thin film organic semiconductor. Lastly, Chapter 7 describes ongoing efforts to characterize the effect of functionalizing MoS2 with chiral molecules. The work presented here represents a significant advance in experimental probes of chemical interfaces and in our understanding of the ultrafast behavior of modified MoS2.