Hybrid buses are widely seen as an important element in reducing carbon dioxide and noxious emissions. These buses are now starting to be seen in significant numbers in some city bus fleets. However, whilst some designs have been successful, others have been plagued by battery problems and failure to achieve predicted fuel savings in service. This has sometimes been caused by lack of understanding by both the vehicle designer and battery supplier, who generally have little experience in each other's areas. This paper looks at some key design issues that need to be addressed in developing a successful hybrid bus battery pack design.