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Among various industrial tomography modalities, electrical capacitance tomography (ECT) is the most mature and has been used for many challenging applications. ECT is based on measuring very small capacitance from a multi-electrode sensor and reconstructing the permittivity distribution in a cross section of an industrial process. Compared with other tomography modalities, ECT has several advantages: no radioactive, fast response, both non-intrusive and non-invasive, withstanding high temperature and high pressure and of low-cost. Because of very small capacitance to be measured (much smaller than 1 pF) and the “soft-field” nature, ECT does present challenges in capacitance measurement and solving the inverse problem. The latest AC-based ECT system can generate online images typically at 100 frames per second with a signal-to-noise ratio (SNR) of 73 dB. Examples of industrial applications include gas/oil/water flows, wet gas separation, pneumatic conveyors, cyclone separators, pharmaceutical fluidised beds, and clean use of coal by circulating fluidised bed combustion and methanol-to-olefins conversion. During this tutorial, ECT is discussed from principle to industrial applications, together with demonstration of an AC-based ECT system.