The idea of combining multiple image modalities to provide a single, enhanced picture offering added value to the observer or processor is well established, but the technology to realise it is somewhat less mature. In the past few years computing power has advanced sufficiently to finally enable affordable, real-time image fusion systems to become a reality and the field has started to move out of the research laboratories and into real products. Although algorithmic techniques for fusing images are now well known and understood, challenges remain with regard to exploiting different sensor modalities, robustness to environmental and operational conditions and proving performance benefit, to name but a few. This paper provides a broad review of the field of image fusion, from initial research published to the latest technology being developed and systems being deployed. Particular emphasis is placed on image fusion developments that have been made for the military community, which were mainly designed to exploit low light devices and thermal imagers. Wider applications of image fusion are also considered as well as all of the main technologies required to produce real-time image fusion systems. A summary of current and near-term products is given, as well as the latest research trends and end-user analyses reported to date.