The work presented in this thesis explores multiple different systems with one overarching theme: a powerful new approach to understanding reactive surface dynamics. Reactions at surfaces–or more broadly speaking, interfacial chemistry–are ubiquitous in the world. From the development of materials that protect spacecrafts upon atmospheric reentry, to semiconductor fabrication for electronic devices, to heterogeneous catalysts used for industrial processes, understanding and controlling interfacial reactivity is crucial to science and technology. At a fundamental level, these three applications critically depend on the gas-surface interactions of O2 with graphite, O2 with gallium arsenide (GaAs), and N2 with ruthenium, respectively. Utilizing a unique and custom built ultra-high vacuum (UHV) chamber that combines a supersonic molecular beam with a scanning tunneling microscopy (STM), the reactive evolution of each of these three interfaces after exposure to energy- and angle-selected gas molecules is visualized to obtain an understanding of the chemistry and physics that govern their behavior.In the case of O2 reacting with a well-characterized form of graphite–highly oriented pyrolytic graphite (HOPG)–the impinging O2 energy and angle, as well as the HOPG surface temperature, affect the subsequent reaction between oxygen and carbon in ways that are both kinetically and morphologically distinct. Similarly, the oxidation of GaAs(110) with O2 proceeds by two kinetically and morphologically distinct mechanisms: the nucleation and subsequent growth of spatially heterogeneous oxide islands and spatially homogeneous random oxidation leading to layer-by-layer growth. Lastly, increasing the kinetic and vibrational energy of impinging N2 on a Ru(0001) surface activates dissociative adsorption on otherwise nonreactive terrace sites, and the resulting spacing and orientation of the nitrogen atom pairs provide critical information about the energy transfer and potential nonadiabatic nature of the dissociation event.