Fundamental understanding and control of quantum phenomena on unprecedented length and time scales are essential for proper development of next generation devices. Recent advances in the synthesis of atomically thin layers of van der Waals solids such as graphene, boron nitride, and transition metal dichalcogenides (TMD) open up possibilities to success, for example, in computing, information and energy technology. Related to photonics and optoelectronics applications monolayer TMDs have potential for increasing the capabilities of conventional semiconductors by broad absorption spectrum, i.e., from near-infrared to the visible region. In this proposal, we will study the light matter interactions in monolayer TMDs and their heterostructures with emphasis on strong excitonic effects, and spin- and valley-dependent properties. To this end, we will develop model Hamiltonian techniques, which in conjunction with density functional theory based calculations will provide new insight in the light matter interactions in monolayer TMDs. The overarching goal of this proposal is to achieve understanding of novel quantum phenomena in monolayer TMDs in particular how heterostructuring, defects, and strain intertwine to produce interesting physical properties. The work proposed here will lead to major advances in understating how defects, heterostructuring, and strain modify the properties of 2D materials, resulting in novel quantum phenomena.