The aim of this thesis is to compare and analyse the introduction of renewable fuel in the public transport sector, focusing on the challenges and opportunities encountered by involved stakeholders on the regional and local levels. The results contribute to answering three research questions: 1) How do organisational factors and local and regional contextual factors influence the introduction of renewable fuel? 2) What are the challenges and opportunities of using green public procurement as a policy tool to introduce renewable fuel in the public transport sector? 3) How do the challenges and opportunities regarding the introduction of renewable fuel differ depending on the type of renewable fuel? Four papers are included in the thesis. Paper I compares and analyses how factors identified in green public procurement research (strategies, requirements, cost, size and knowledge) influence the choices made when introducing renewable fuel in two Swedish regions. The findings show that the influence of the factors is highly case-specific and that differences in their strategic approach caused regions to express requirements for fuel differently in tender documents. Functional requirements were used by the public authorities to increase the share of renewable fuel in a cost efficient way and at the same time allow room for flexibility and leave more control to the operators. Specific requirements were strategically used to create local markets for biogas, which poses higher demands on political backing, knowledge by the public authorities, and an acceptance of increased costs. These findings were further elaborated in paper II, where introduction of renewable fuel in ten more Swedish regions was studied. The results confirmed to a large extent the challenges and opportunities from paper I. Further, regions that had introduced another renewable fuel than biodiesel had either used specific requirements or introduced the fuel in publicly operated bus services. The scope of paper III complements the findings by looking more in detail at how environmental requirements have been expressed, by performing a content analysis of Swedish tender documents. The results show that size of the procurement and type of traffic influence how environmental requirements are set. Further, both ambitious functional requirements and specific requirements for fuel are more common in large tenders in city traffic – this confirms and exemplifies the importance of context when renewable fuel is introduced through public procurement. In paper IV, the focus is solely on the introduction of electric buses by comparing experiences in Sweden and England. It was concluded that most challenges are case-specific on the city level, for example, passenger demand and bus route characteristics, but also financial and regulatory support from the national government can have an influence. Additionally, the relationship and division of roles between involved stakeholders are central to overcome challenges in all cities. Overall, this thesis concludes that green public procurement can be successful for introducing renwable fuel. By expressing requirements differently in the tender documents, public authorities have been able to influence the introduction to a varying degree which has led to different challenges and opportunities for the involved stakeholders as well as different outcomes for renewable fuel. Nevertheless, introduction of emerging technologies (eg. electric buses) was shown to be a challenge when public transport was procured. Alternative introduction strategies were seen for electric buses, such as test projects, introduction under current procurement contracts, increased collaboration between stakeholders and more responsibility to cover for uncertainties taken by the public authorities. In summary, the challenges and opportunities of introducing renewable fuel are highly case-specific and strongly associated with the specific fuel in question.