Transition metal dichalcogenides are atomically thin two-dimensional layers with intriguing optical properties. The four famous ones with closely related properties are MoS2, MoSe2, WS2, and WSe2. Their monolayers show strong excitonic emission in the visible range (exciton binding energies ~500meV); their bulk forms are indirect-gap semiconductors. We are interested in their optical properties, including exciton-phonon coupling, and in their interactions in heterostructures with other layers.