High frequency ignition (HFI) and conventional transistor coil ignition (TCI) were investigated with an optically accessible single-cylinder research engine to gain fundamental understanding of the chemical reactions taking place prior to the onset of combustion. Instead of generating heat in the gap of a conventional spark plug, a high frequency / high voltage electric field is employed in HFI to form chemical radicals. It is generated using a resonant circuit and sharp metallic tips placed in the combustion chamber. The setup is optimized to cause a so-called corona discharge in which highly energized channels (streamers) are created while avoiding a spark discharge. At a certain energy the number of ionized hydrocarbon molecules becomes sufficient to initiate self-sustained combustion. HFI enables engine operation with highly diluted (by air or EGR) gasoline-air mixtures or at high boost levels due to the lower voltage required. Key reasons for the superior performance are the much larger activated volume (the streamers being several millimeters long) and the possibility to extend the ignition duration into the millisecond range. As a result more advanced combustion strategies can be realized with less cycle-to-cycle variations.In the present investigations, sophisticated optical methods were used in-situ, namely UV-VIS spectrography for the detection of different radicals in a time-resolved manner and high-speed recording for additional spatial resolution. These results were compared with TCI. Moreover investigations in a heated pressurized chamber were performed to gather further information about the propagation of streamers depending on pressure, temperature and voltage.