Quantities that need to be measured in gas fluidized-bed systems include solids volume concentrations, solids velocities and solids mass flows, the vertical and horizontal distribution of solids inside the system, the lateral distribution of the fluidizing gas, temperatures and gas concentrations. In the present paper an overview is given on available measuring techniques. In the first section techniques for industrial routine measurements are discussed. These are mainly temperature and pressure drop measurements. Practical applications and also the limitations of these techniques are outlined. In the second section more sophisticated techniques for local measurements inside fluidized bed systems, which have already proven their suitability in large-scale industrial reactors, are dealt with. Examples include suction probes for measurements of local solids mass flows, heat transfer probes for the detection of defluidized zones and solids flows inside fluidized-bed reactors and capacitance probes for solids concentration and velocity measurements under high-temperature conditions. The third section finally presents advanced techniques which are either still under development or which are particularly intended for academic investigations of basic fluidization phenomena. Examples include sensor techniques, imaging and tomographic methods.