Energy saving in general and less polluting vehicles in specific, become more and more urgent topics. One reason is that, in a world where the demand for fast transportation is increasing, the risk of global warming is a fact. Hybrid Vehicles (HV:s) are proposed as a more environmentally friendly candidate than conventional vehicles. Nowadays, there are numerous different types of HV:s and the components can, in theory, be sized in infinite ways. There is no simple answer to how to choose driveline configuration and size components in a HV. This paper describes one method, Driveline Synthesis (DS), that systematically presents a suitable driveline, on the basis of demands and conditions. Examples of demands are driving cycle and emission free zones. Some conditions are fuel price, tax on pollution and discount rate. The most suitable driveline is defined as the most cost effective. Total cost is defined as the sum of: cost of components, fuel cost, cost of external energy and cost of pollution. Genetic algorithms are used as an optimization method.Two major types of drivelines are compared in a case study, a conventional bus with a diesel engine and automatic transmission versus a series hybrid bus with different types of primary power units (diesel engine or fuel cell) and storage devices (super capacitor or NiMH battery). DS gives reasonable answers but needs further validation and development. One conclusion from the work is that the most suitable driveline configuration depends very much on demands, conditions and present technology, i.e. HV:s are only preferable to conventional vehicles under special circumstances.