Vesselin Petkov and Luchezar Stoyanov Sojourn times, singularities of the scattering kernel and inverse problems (510K, Postscript) ABSTRACT. We study the scattering kernel related to a perturbation caused by a bounded (non-convex) obstacle K. The scattering kernel is determined by the Fourier transform of the scattering amplitude and its singularities are related to the sojourn times of scattering rays. The purpose of this survey is to describe several results in obstacle scattering obtained in the last twenty year concerning the sojourn times and to study some inverse scattering problems.