This publication presents the state of the art in the design approaches used to achieve defence in depth in pressurized water reactors, pressurized light water reactors, cooled heavy water moderated reactors, high temperature gas cooled reactors, sodium cooled and lead cooled fast reactors, and non-conventional designs within the small and medium sized reactors (SMRs) range. Particular attention is given to those approaches that eliminate accident initiators or prevent accident consequences by incorporating inherent and passive safety features and passive systems into the safety design concepts of such reactors. The information presented in this publication could be used for assessment studies of innovative nuclear energy systems involving SMRs, as well as in evaluating the overall technical potential of innovative SMRs in Member States.