A recent addition to low-dimensional materials are monolayer transition metal dichalcogenides (TMDs), such as MoSe2 and WSe2, with an atomically thin, honeycomb-like lattice. The presence of a large, direct band gap makes monolayer TMDs well suited for optics. In addition, the layered structure of TMDs can be exploited to engineer tailored optical and electronic properties by stacking different 2D crystals into what is known as a van der Waals (vdW) heterostructure. The charge carriers in monolayer TMDs are expected to have strong Coulomb interactions due to strict 2D confinement. This, for example, leads to an optical spectrum which is dominated by tightly bound electron-hole pairs or excitons. Such strongly bound excitons are ideal for studying strong coupling between light and matter.