This publication reports the results of a recent IAEA coordinated research project on industrial process gamma tomography, a non-intrusive technique capable of measuring the phase distribution inside multiphase equipment without disturbing its normal operation. This makes it invaluable in the investigation of various processing units within multiphase systems, such as distillation columns, fluidized beds and catalytic reactors. This publication will serve as a resource for students and specialists in the fields of chemical engineering and radioisotopes. The accompanying CD-ROM includes software for image reconstruction which, together with the written publication, can be used as foundational lecture material in the transfer of knowledge to developing countries.