In this technical review we give an introduction to optical spectroscopy for layered materials as a powerful, non-invasive tool to access details of the electronic band structure and crystal quality. Potential applications in photonics and optoelectronics are based on our understanding of the light-matter interaction on an atomic monolayer scale. Here atomically thin transition metal dichalcogenides, such as MoS2 and WSe2, are model systems for layered semiconductors with a bandgap in the visible region of the optical spectrum. They can be assembled to form heterostructures and combine the unique properties of the constituent monolayers. We review the working principles of micro-photoluminescence spectroscopy and optical absorption experiments. We discuss the physical origin of the main absorption and emission features in the optical spectra and how they can be tuned. We explain key-aspects of practical set-ups for performing experiments in different conditions such as variable temperatures or in applied magnetic fields and how parameters such as detection spot size and excitation laser wavelength impact the optical spectra. We describe the important influence of the direct sample environment, such as substrates and encapsulation layers, on the emission and absorption mechanisms. A survey of optical techniques that probe the coupling between layers and analyse carrier polarisation dynamics for spin- and valleytronics is provided.
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