The demand for long-term, sustained, reliable data and derived information on climate and its changes has never been greater than today. Long-term, well-calibrated, global observations of Essential Climate Variables (ECV) such as air temperature, precipitation, and sea-surface temperature are critical for defining the evolving state of the Earth's climate. Observing systems routinely collect much of the required data covering 49 ECVs, and significant progress has been made in coverage and technological capability over the two decades since the Second World Climate Conference. However, many key regions and climatic zones remain poorly observed, and gaps are widening in some cases. Supporting infrastructures for data stewardship and analysis are largely in place but require strengthening, while those for linking with socio-economic data and for providing user-oriented information services require more substantial development. The current capabilities are summarized, and further actions are identified to ensure that climate observation activities more fully meet the needs of science and society. The Global Climate Observing System (GCOS) was established in 1992 with the goal of providing comprehensive information on the total climate system, involving a multidisciplinary range of physical, chemical and biological observations of the atmosphere, oceans and land. GCOS is a "system of systems" that builds on the climate-relevant components of existing observing systems, and relies almost entirely upon national efforts to maintain and enhance those systems. Contributing systems include the World Meteorological Organization Global Observing System (GOS) for meteorology, its Global Atmosphere Watch (GAW) for atmospheric composition, the Global Ocean Observing System (GOOS), led by the United Nations Educational, Scientific and Cultural Organization (UNESCO) Intergovernmental Oceanographic Commission (IOC), and the Global Terrestrial Observing System (GTOS), led by the Food and Agriculture Organization of the United Nations (FAO). GCOS itself is the climate observing system within the Global Earth Observation System of Systems (GEOSS) developed under the auspices of the Group on Earth Observations (GEO). The established in situ networks and spacebased components must be sustained and operated with ongoing attention to data quality in accordance with the GCOS Climate Monitoring Principles; enhancements must be made for some types of observations; the exchange of observations and delivery of data and information to users must be ensured; reprocessing and reanalysis must be strengthened; and national and international coordination must be improved. The consequence of not meeting these requirements would be to seriously compromise the information on, and predictions of, climate variability and change. Detailed information on GCOS and the datasets that are produced as a result of GCOS observing activities can be found at the Global Observing Systems Information Center (GOSIC). © 2010 Published by Elsevier.