1.1 Objective and organization of chapter In order to study the hydrodynamic behaviour of large flotation machines, the radioactive tracer technique has been used to measure a number of internal characteristics such as: • Residence time distribution (RTD) of liquid, solid and gas, in industrial cells and columns. Actual mean residence time evaluation. • Mixing regime in single cells, banks of cells and pneumatic columns. • Froth mean residence time of liquid, floatable and non-floatable solids. • Mixing time and internal pulp circulation in large industrial self-aerated cells. • Gas holdup and gas residence time distribution in flotation machines. • Direct measurement of gangue entrainment. • Industrial flotation cell scanning with gamma ray. • Pulp flowrate distribution in parallel flotation banks. • Flotation rate distribution.