The latest developments and results in an experimental study aimed at the design and implementation of an innovative ignition system for S.I. engines are shown. The system is based on the free discharge of energy at microwave frequency. Due to the characteristics of free discharges at high frequency, the system allows long duration discharges with low electrode erosion, even when high energy levels are used and should assure a good energy transfer to the charge. Moreover, the presence of a microwave field inside the cylinder should enhance the combustion process for the well known principle of the electromagnetic stimulation of the combustion. An objective is the increase of thermal efficiency and the reduction of exhaust emissions, thanks to a fast and regular combustion of very lean or diluted (with E.G.R.) mixtures. An other objective is the improvement of the combustion of two cycle engines at low load.