Linear friction welding (LFW) is a solid state joining process, in which a joint between two metals can be formed through contact of a plasticised layer at the interface of adjoining specimens. In order to form the joint, the two specimens, one oscillating and one stationary, are brought together under force. The friction between specimens causes heat generation, which along with the axial force cause the material at the weld interface to plasticise. Oxides and other impurities are removed from the specimen surfaces during welding, and this allows metal-to- metal contact between the plasticised surfaces, which in-turn allows a joint to form.Currently, LFW is established as a niche technology for the fabrication of titanium alloy bladed disk (blisk) assemblies in aero-engines, and is being developed for nickel based superalloy assemblies. However, interest is growing in utilising the process in a wider range of applications that employ a wider range of materials. A critical review of the process does not currently exist, and it is hoped that one that demonstrates the potential of the technology to join a wide range of engineering materials, will help it grow into more than a niche technology and penetrate into new fields.This review will cover all published work conducted to date on LFW, as well as providing a brief comparison with rotary friction welding, which is a fundamentally similar process. The review will firstly describe the basics of the process and the fundamental aspects of operating a LFW machine. It will then describe the different materials that have been welded using the process, and give detailed descriptions of the microstructural changes that take place within the weld region and how they impact on the performance of the joint. In addition, the evolution of residual stresses produced by the welding process will be described. The review will conclude with an analysis of the areas poorly covered by the publically available literature, and suggest areas for future research into the subject.