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Isothermal vapour-phase epitaxy (ISOVPE) was the earliest process to be developed for the fabrication of epitaxial mercury-cadmium telluride layers. Although the process was somewhat neglected in the previous period, mostly because of the appearance of metalorganic chemical vapour phase epitaxy and molecular-beam epitaxy, the results of the most recent investigations show that the application of this method can be expected in the second generation of photovoltaic (PV) or metal-insulator-semiconductor (MIS) detectors in the focal plane. This review paper presents the development of ISOVPE giving a complete theoretical model of the process, which is based on fundamental results in the field of phase diagrams of the (Hg,Cd)Te the interdiffusion of Hg and Cd, gas-phase transport, etc. Using the results of this theoretical model, a production system for ISOVPE was designed, in which the deposition process, the high-temperature annealing for composition homogenization and the low-temperature annealing for the achievement of the desired epitaxial-layer doping are all performed in the same cycle. Finally, the basic electrical and optical characteristics of ISOVPE layers are analysed. It is shown that in applications of the fabrication of PV and MIS detector arrays a full affirmation of this technology can be expected.