  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.
