High-temperature optical microscopy is an essential in situ characterisation and monitoring technique with wide applications in different areas of materials science. The devices used include commercial available instruments, known as heating microscopes, and custom-made devices, usually called "high-temperature processing microscopes＇＇ or "thermo-optical instruments＇＇. The different areas of applications of high-temperature optical microscopy are discussed on the basis of practical examples drawn from the literature. Besides the classical use of the technique to study the melting and softening behaviour of glass, slags, ashes and other silicate and ceramic materials, this review covers alternative applications, in particular the use of heating microscopes as "optical dilatometers＇＇ to investigate the sintering kinetics of powder compacts. In this regard, the advantages of the technique over conventional dilatometry are emphasised. A variety of custom-made devices is described, developed to investigate particular problems, such as delamination and curling of laminate composites during densification, cosintering of multilayer metal-ceramic and ceramic-ceramic systems, and wetting behaviour of liquid phases on rigid substrates. As a particular example of such a custom-made equipment, a novel, multi-purpose high-temperature processing microscope is described, and its application potential, which is well beyond that of commercial devices, is outlined. This instrument is unique in that it combines both vertical and horizontal sample observation capability, as well as the possibility to investigate samples of relatively large sizes (65 mm(3)), i.e. about 10 times larger than those suitable for commercial heating microscopes.