Due to their inherent confinement of charge carriers, transition metal dichalcogenide (TMD) monolayers present an optimal platform for observing excitonic phenomena. A large spin-orbit coupling, in combination with a broken inversion symmetry in the monolayer limit, leads to novel spin-valley physics. The 2D nature of the monolayers leads to large exciton binding energies, causing excitonic effects to dominate the strong light-matter interaction even at room temperature. Advances in the assembly of van der Waals (vdW) heterostructures allow the implementation of TMDs into complex, layered structures, where materials such as few-layer graphene and hBN can act as gate electrodes and insulating dielectric barriers. Furthermore, due to their vdW crystal nature, vdW heterostructures can be easily integrated into existing device designs, such as silicon-based photonic integrated circuits, which hold promise for future applications. In this thesis, we investigate delocalized interlayer excitons (IXs) in TMD hetero-bilayers and localized defect-bound excitons in MoS2. In the first part, we give a detailed introduction to state-of-the-art vdW heterostructure assembly processes and analyze the samples’ interface and optical quality. We continue discussing delocalized interlayer excitons that form due to a type II band alignment of a TMD hetero-bilayer. Large exciton binding energies on the order of 100 meV and a reduced electron-hole wavefunction overlap, leading to lifetimes up to μs, make the system ideal for the exploration of many-body phenomena. We demonstrate several criticalities in the IX phase diagram that we interpret as signatures of a degenerate exciton gas. Furthermore, we present back focal plane imaging of the IX emission and determine an almost entirely in-plane optical transition dipole with weiterlesen