The discovery of monolayer two-dimensional semiconductors of atomic-scale thickness presents a new two-dimensional landscape in which to play with the interaction between light and matter. These nanomaterials at the extreme limit of surface-to-volume ratio exhibit rich optical phenomenology such as layer dependent bandgaps and degenerate, but distinct, valley-polarized excitonic states. The unique features of atomically-thin materials suggest that these layered systems can be exploited to achieve new regimes of light-matter interactions. In this presentation, I will discuss efforts to control the interaction of monolayer semiconductors with light using both top-down nanopatterning and photonic device integration. In particular, I will describe the emergence of spin-polarized exciton-polariton quasi-particles in monolayer semiconductors embedded in a photonic microcavity. Cavity enhancement of optical interactions results in modified dynamics of these coherent light-matter states. Examples will illustrate how optical and quantum phenomena can be rationally designed in monolayer semiconductors, suggesting exciting potential for novel hybrid quantum systems or opto-electronic applications harnessing the unique properties of low-dimensional nanomaterials.