The demand for electrochemical energy sources is nowadays extremely large and it addresses very different application, from small portable devices, over electric vehicles, to large stationary applications. The requirements for the electrochemical energy sources are therefore extremely various in terms of cost, specific power and energy, cycle life, safety. In spite of the large variety of electrochemical energy storage systems available today they may not fulfil all of the requirements requested. The need of achieving both high energy density and power density has been pointed out in the last decade and, among the different possible approaches, the hybridization of two types of electrochemical energy storage devices, rechargeable battery and electrochemical double layer capacitor, has been strongly investigated. This work reviews the different approaches to the hybridization, such as internal and external, serial and parallel and provides a collection of today's achievements.