My research interests have been the development and application of theoretical methodologies for calculations on defects in solids and at surfaces, with particular emphasis on embedded cluster techniques. A significant part of my efforts were focused on atomistic theory of the geometric and electronic structure of point defects in insulators and semiconductors, and on the understanding of the mechanisms of electronic and ionic processes in ionic crystals. I developed theories of the mechanisms of photo-induced processes at ionic surfaces and models of self-trapped excitons and hole polarons in solids and at surfaces, especially in relation to ultrafast processes induced by electronic excitation. During the last several years I was strongly involved in the development of theoretical models of atomic force microscopy on ionic surfaces. I am a co-author of one book and a co-editor of another book on theoretical methods of defect studies in solids.