This is chapter 6 of the State of Environmental Science in Svalbard (SESS) report 2021. Svalbard Integrated Arctic Earth Observing System (SIOS) is an international consortium of currently 26 member institutions which develops and maintains a regional observing system in Svalbard and surrounding waters. SIOS brings together the infrastructure and data of its members into a multidisciplinary network dedicated to answering Earth System Science (ESS) questions related to global change. The ‘SIOS Core Data’ (SCD) are composed of long-term data series collected by SIOS partners, fulfilling defined criteria: (1) relevant to answer key ESS questions, (2) available to interested parties according to advanced (‘FAIR’) data management principles and (3) data collection to be guaranteed by members for a minimum of 5 years. The first set of SCD variables has been identified by the Science Optimisation Advisory Group in cooperation with the Research Infrastructure Coordination Committee and scientific experts. Many SCD variables are derived from the list of Essential Climate Variables defined by the Global Climate Observing System and are described using WMO standards and the Global Change Master Directory keywords, thus following earlier standardisation efforts. SCD variables are critical for characterising the climate system and its changes in the Arctic, and answering key ESS research questions prioritised by the SIOS community. SIOS activities related to SCD are in line with Sustaining Arctic Observing Networks’ (SAON) Roadmap for Arctic Observing and Data Systems process. SIOS core data are made freely available through the SIOS Data Access Portal. The datasets cover mainly physical entities like geophysical, meteorological, or oceanographic data. They allow for example the determination of mass and energy flows across Svalbard, which enables better understanding of the archipelago’s role within the Earth System.