The fundamental principles of electrical capacitance tomography (ECT) are presented. In particular, the recent developments in the use of a back-projection algorithm applied to solve an inverse problem are discussed. Experimental results obtained for monitoring a pneumatic conveying system and bubbling fluidization are reviewed and revised data are presented. The performance of the ECT is assessed in quantitative and qualitative terms. The use of ECT to provide novel on-line measurement information suitable for modelling or control purposes is illustrated.