Design and technology development for small and medium sized reactors (SMRs) is ongoing in many countries, and there are growing expectations of increased support from the IAEA to interested Member States in the definition of common technology and infrastructure development needs and in the coordination of international R&D efforts for such reactors. This publication presents a variety of innovative water cooled, gas cooled, liquid metal cooled and non-conventional SMR designs developed worldwide, and examines the technology and infrastructure development needs that may be common to several concepts or lines of such reactors. This publication also gives an updated definition of small reactors without on-site refuelling and a preliminary review of the passive safety design options for SMRs.