High-pressure homogenisation is a commonly used technique to produce emulsions with droplets in the micro to nano scale. Due to the flow field in the homogenizer, stresses are transferred to the interface between droplets and continuous phase. Cohesive forces within droplets interact with external stresses. To exceed the cohesive forces, high process pressures are necessary, which might cause a complex flow pattern and large flow velocities. Additionally, the pressure drop can induce cavitation. Inline measurements are a challenge, but necessary to understand droplet break-up in a high-pressure homogenizer. Recently, different optical methods have been used to investigate the flow conditions as well as the droplet deformation and break-up in high-pressure homogenisation, such as high speed imaging, particle and micro particle image velocimetry. In this review, those optical measuring methods are considered critically in their applications and limitations, achievable results and further developments.