Over the world, the pollution norms are becoming more and more stringent. The European tolerance for NOx emission of diesel light duty vehicles has been divided by 3 between 2000 and 2009.The Euro 6 norm will be even more strict and diesel ICE based vehicles have to deal with this constraint. HEVs can be a solution for automotive industry to reduce emission and respect more easily the current and ongoing norms. On the other hand, safety must be kept at high level in the HEVs. The interaction between the regenerative braking for example and the different systems for safety (ABS, ESP, ...) has to be carefully considered. To achieve the consumption and emission target without generating safety problems or prohibitive additional cost a global design optimization is strongly recommended. Three interdependent dimensions should be taken into account in the optimization approach: i) the drive train architecture, ii) the components type and size iii) the energy management. A two level, model based, optimisation methodology could be established. The lower level deals with the energy management optimization using different methods such as dynamic programming or the Pontryagin minimum theory. This level is activated when the vehicle architecture and the different components are known. The upper level operates the variations in the components design parameters using iterative methods or genetic algorithms for example. The global design should also consider either the typical use profile or the most stringent use specification. According to the performance acceptability by the end user, the design of the optimal solution can be significantly changed.