This thesis presents observational and theoretical studies of the Gamma-ray bursts. Gamma-ray bursts are thought to be associated with events of violent gravitational collapse either at the end of the evolution of massive stars or after merging of neutron stars and black holes. The mechanism responsible for the emission of the gamma-rays as well as associated radiation from radio to X-ray is not yet understood. In the scope of this thesis observations are used to critically test the gamma-ray burst models. New models necessary to reach agreement with the observations are discussed.