Microemulsions have a wide range of applications from oil recovery to synthesis of nanoparticles. The first implementation of water-in-oil (w/o) microemulsions for synthesis of nanoparticles was introduced in 1982 and concerns nanoparticles of noble metals for catalytic uses. Since this time, the method has been employed quite extensively in the field of catalysis, from room-temperature reactions such as butene isomerisation to high-temperature reactions such as catalytic combustion of methane. The present review paper will introduce the term microemulsion with emphasis on the microemulsion properties essential for heterogeneous catalyst preparation followed by a general description of the mode of catalytic materials prepared from microemulsions. Several examples of the use of these materials in heterogeneous catalysis are then described. It is shown that nanoparticles obtained from microemulsions have specific properties with respect to size, size distribution and surface structure. Consequently, these particles are in some cases superior regarding catalytic activity and/or selectivity. (C) 2004 Elsevier B.V. All rights reserved.