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The merger of intense coulombic interactions with a unique 2D band structure in atomically thin semiconductors gives rise to rich excitonic behavior that opens new scientific and technological opportunities but occurs on experimentally challenging ultrafast timescales and ultrasmall length scales. By combining powerful optical techniques such as nano-optical and low-temperature photoluminescence spectroscopies, deep insight can be gained at these scales on how the excitonic properties of 2D materials produce, for instance, new quantum functionalities. Recent nano-optical studies by our team have revealed a new mechanism by which strain localizes excitons on nanometer scales at room-temperature, providing key insight into single-photon emitters in WSe₂. And low-temperature studies have demonstrated that optical excitation of an electron-hole plasma and its ultrafast relaxation enhances the formation of multiexcitons, which are known emitters of entangled photon pairs. These studies elucidate structure-property relationships and many-body dynamics that are unique to 2D semiconductors and important for harnessing these materials for next-generation quantum optoelectronics.