Using natural climate archives such as ice cores, lake sediments, stalagmites and tree rings, but also historical documents, we look back several thousand years and investigate how natural driving factors (the sun and volcanoes) and anthropogenic disturbances (greenhouse gases, land use, aerosols) have changed the climate and how ecosystems react to such changes. The comprehensive reconstruction of the climate of the past is a core competency of the Oeschger Centre. The long proxy data and measurements series in Europe and in Switzerland allow climate reconstructions of a quality unique in the world. They provide detailed input into the long-term diagnosis of atmospheric dynamics (pressure, precipitation and temperature), the causes of variations in climate and the statistics of extreme events.