The next generation of 40 m class Extremely Large Telescopes (ELTs) are currently under construction. These vast instruments will enable new discoveries in all areas of astronomy and push forwards the boundaries of human knowledge. They will look further back in space and time to explore the early universe and shed light on unanswered questions such as dark matter and dark energy. They will discover and characterise extra-solar planets and potentially find habitable, or even inhabited, worlds. To fulfil these ambitious objectives these giant telescopes will be equipped with highly sophisticated adaptive technologies in order to counteract the detrimental effects of the Earth’s atmosphere. The characterisation of atmospheric optical turbulence is critical for advanced optical astronomical observations. This includes using data from Adaptive Optics (AO) systems, dedicated auxiliary instrumentation as well as forecasts from numerical models of the Earth’s atmosphere. Exploiting this hybrid approaches enables us to determine the vertical profile of the turbulence strength, velocity, outer scale as well as the local turbulence contained in the telescope dome. This detailed knowledge is vital for wide-field AO systems which are particularly sensitive to the vertical structure of these atmospheric parameters and for highly-complex systems such as extreme AO, where the varying atmospheric parameters have a significant impact on performance. Atmospheric turbulence characterisation is therefore required for modelling, monitoring and optimising AO instrumentation, enabling efficient operation of current sophisticated instrumentation systems as well as the future Extremely Large Telescopes.