Seismic instrumentation systems are important elements for plant safety. They provide crucial information for the assessment of safety required to restart a plant after a shutdown caused by an earthquake. This publication presents information and experience related to the development of seismic instrumentation systems and the utilization of data recorded by them, based on recent seismic events. The publication is intended to be a reference for developing effective post-earthquake actions/procedures. It describes the typologies of seismic instrumentation systems and their application for obtaining 'earthquake levels' and 'damage indicating parameters' that are used to predict the extent of seismic damage caused by the recorded earthquake motions. It complements IAEA Safety Standards Series No. NS-G-1.6, Seismic Design and Qualification for Nuclear Power Plants, and Safety Reports Series No. 66, Earthquake Preparedness and Response for Nuclear Power Plants, as a technical publication relevant to seismic safety for new and existing nuclear installations.