Gamma ray bursts are one of the most enigmatic cosmic objects, releasing enormous amount of energy mostly in the keV to MeV energy range in our cosmos within a very short time. Their short durations and unpredictable locations at the sky make their observations very difficult. The MAGIC telescope is the world largest imaging atmospheric Cherenkov telescope. The goal of the project was to contribute to the understanding of these phenomena, in particular to find the upper energy endpoint. With the development of the active mirror control and new focussing methods, I have strongly contributed to the goal. The short repositioning time of the telescope and the low energy threshold allow now to study GRBs at energies close to 100 GeV sometimes in realtime.
During the first year of operation nine GRBs were observed by MAGIC. My thesis deals with the analysis of this data. In two cases the observations started >40s after the burst onset, while the burst was still active. These are the first prompt emission observations of an GRB by Cherenkov telescopes. No evidence of very high energy emission from the nine observations was found and upper limits at energies ~100 GeV, exceeding past measurements by three orders of magnitude, are given.