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propulsion system. Although these technologies have been deployed in operational bus fleets, there remain a number of barriers to widespread deployment. London has one of the most comprehensive and busiest public transport networks in the world, operated by Transport for London (TfL). There are over 9000 buses in operation [12], which are estimated to account for 21% of the CO2 emissions in London [7], 63% of NOx and 52% of PM10 particulate emissions [13]. It is reported that the TfL bus fleet carries 6 million passengers each working day, which the number of bus passenger journeys grew by 64% between 2000 and 2013 and is continuing to increase [14]. The Greater London Authority (GLA) has introduced a number of strategies in an attempt to reduce emissions from buses, part of which is the London hybrid bus project which aims to replace the conventional bus fleet with diesel hybrid buses [7, 15]. This is to be furthered with the introduction of the ultra- low emissions zone (ULEZ) in 2020, which, amongst other targets will require all 3000 double- decker buses operating in the ULEZ to be diesel hybrid and all 300 single decker buses to be zero emissions [8, 9, 16]. Since 2004, a number of technologies have been deployed as part of the operational bus fleet, as shown in Figure 2, as a means of reducing emissions. London has been used as a case study throughout this chapter due to both the comprehensive bus network and the operational deployment of new technologies. Within this chapter, the development of low emission bus propulsion technologies will be dis- cussed, through the evolution of diesel to diesel hybrid buses and onto the development and deployment of battery electric and FC buses. The aim is to outline the benefits of such tech- nologies and the barriers that exist to their widespread implementation from both a technical and economic perspective. Part 2 discusses the implementation of diesel electric hybrid buses Figure 2. Timeline of the milestones for the London low emission bus deployment. Development of Bus Drive Technology towards Zero Emissions: A Review http://dx.doi.org/10.5772/68139 35