This article offers a comprehensive overview of research on fuel reforming in internal combustion engines (ICE). It includes a historical perspective of research in this field, a discussion on the considerations to be made prior to choosing a primary fuel for reforming purposes, and the main processes in fuel reforming. Steam reforming offers a moderate degree of thermochemical recuperation and is applicable to methanol and ethanol feeding. Reforming with air reduces the degree of recuperation, but opens up the use of heavier fuels such as gasoline and diesel fuel. Dry reforming (with CO2) offers the best recuperation but is prone to fast coking. The choice of catalyst and the expected side reactions for each fuel are also discussed. While there is extensive literature on steam reforming catalysts and kinetics at atmospheric pressure, studies at higher pressures and/or on decomposition reactions are very few. The thermodynamics of fuel reforming in ICE and simulation approaches are also discussed. The paper also reports on engineering aspects of fuel reformer design and provides an overview of engines with thermo-chemical recuperation (TCR), fuel supply, and load control strategies in ICE with TCR. In-cylinder fuel reforming as well as application of fuel reforming for performance improvement of emission aftertreatment systems are subsequently discussed. This overview reveals ongoing diverse research activities in the field of onboard fuel reforming. However, several problems, including reformate burning velocity at typical for ICE conditions, in-cylinder behavior of directly injected reformates and particle formation still need to be addressed. A discussion on some of these unresolved issues is attempted herein. (C) 2018 Elsevier Ltd. All rights reserved.