The use of esters as transformer insulation has been gaining increasing interest over the past few years because of their better environmental performance, higher resistance to the influence of moisture and for some particular liquids, higher flash point as well. However, in order to completely replace the traditional mineral oil and be considered as a viable insulating fluid for high voltage equipments, these liquids must have comparable responses to mineral oil under DC,AC, lightning impulse as well as switching impulse waveforms. The study of pre-breakdown phenomenon(streamer activity) under different stresses has proven to be crucial in determining the breakdown process taking place in transformer insulations. To study the streamer characteristics in ester (MIDEL 7131) and mineral oil (Shell Diala Transformer oil), both electrical and optical techniques have been used in this thesis. Significant research was conducted in identifying the parameters for the selection of the optical sensor suitable for this application. Various diagnostic techniques were employed for accurate detection of streamers electrically as well as optically. Streamers were initially studied in the highly non uniform needle-plane geometry under DC and lightning impulse stress. This configuration represents the situation of a defect in the transformer. Pressboard was introduced at the needle tip to simulate the effect of triple point as a source of discharge. Experiments conducted in Lodz University of Technology proved to be crucial in understanding the optical detection of streamers under this configuration. The main goal of this thesis was to detect streamers originating from transformer winding. For the first time, electrode arrangements involving transformer windings were tested in ester and mineral oil under lightning impulse for streamer activity. The thesis presents the various streamer signals obtained under different configurations and provides important analysis regarding the characteristics of streamers in transformer insulation. The possible effect of spacers between transformer windings and wooden blocks used in transformer coils in also analyzed. The tests performed in this thesis can contribute to the ultimate goal of understanding the difference in streamer propagation in real life electrode arrangements between ester and mineral oil.