Over the last century, there have been reports of high pressure hydrogen leaks igniting for no apparentreason, and several ignition mechanisms have been proposed. Although many leaks have ignited,there are also reported leaks where no ignition has occurred. Investigations of ignitions where noapparent ignition source was present have often been superficial, with a mechanism postulated which,whilst appearing to satisfy the conditions prevailing at the time of the release, simply does not stand upto rigorous scientific analysis. Some of these proposed mechanisms have been simulated in alaboratory under superficially identical conditions and appear to be rigorous and scientific, but thesimulated conditions often do not have the same large release rates or quantities, mainly because ofphysical constraints of a laboratory. Also, some of the release scenarios carried out or simulated inlaboratories are totally divorced from the realistic situation of most actual leaks. Clearly there aregaps in the knowledge of the exact ignition mechanism for releases of hydrogen, particularly at thehigh pressures likely to be involved in future storage and use. Mechanisms which have been proposedin the past are the reverse Joule-Thomson effect; electrostatic charge generation; diffusion ignition;sudden adiabatic compression; and hot surface ignition. Of these, some have been characterized bymeans of computer simulation rather than by actual experiment, and hence are not validated.Consequently there are discrepancies between the theories, releases known to have ignited, andreleases which are known to have not ignited. From this, postulated ignition mechanisms which areworthy of further study have been identified, and the gaps in information have been highlighted. As aresult, the direction for future research into the potential for ignition of hydrogen escapes has beenidentified.