The fluid dynamics in bioreactors is dominated by heterogeneities that are caused by the interplay between the agitated liquid and the gaseous phase that is dispersed in bioreactors to achieve a sufficient oxygen supply and desorption of CO2. These heterogeneities are the major drawback in the reliable design and scale-up or scale-down of bioreactors because they are difficult to predict. One reason for the unpredictability of heterogeneities in bioreactors is the bubble size distribution that depends on the local shear stress as well as the properties of the gas-liquid interfaces. The bubble size distribution influences buoyancy driven flows and with this the overall mixing, the gas holdup distribution as well as the interfacial area – all essential parameter to predict the mass transfer performance. In this research group, new methods are under development to measure and predict the heterogeneities in bioreactors, to enable a more reliable design, scale-up and scale-down. Flow heterogeneities are measured by visualization in transparent bioreactors from the laboratory scale to the industrial scale. For non-transparent reactors, Lagrangian Sensor Particles are under development, to detect flow heterogeneities and Lifelines in multiphase bioreactors.