The aim of this paper is to review the factors already discussed in the literature and identify gaps or issues which seem to require further debate in relation of the introduction of hydrogen in the transport sector. Studies in the academic and grey literature have analysed transport systems with a rather wide range of hydrogen penetration rates, utilisation of the infrastructure, hypotheses on the dynamics of the systems, capital costs of the infrastructure and hydrogen price. Most of the issues which could widen the debate in the literature are related to policy instruments. In particular, more attention should be paid to the policy instruments needed to foster co-ordination among stakeholders, persuade drivers to buy hydrogen vehicles despite the existence of a sparse infrastructure; guarantee investment in the early, possibly loss-making, retail stations and to foster financially sustainable govemment commitments. The effect of limited availability of hydrogen vehicle models on the penetration rates in the literature and the sensitivity of the hydrogen price to taxation from the government are other two issues deserving a more in-depth discussion. (c) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.