The decision to produce a FourthAssessment Report (AR4) was taken by the 19th Session of the Intergovernmental Panel on Climate Change (IPCC) in April 2002. The Working Group II Report has twenty chapters. The core chapters (3 – 16) address the future impacts of climate change on sectors and regions, the potential for adaptation and the implications for sustainability. Chapter 1 looks at observed changes and Chapter 2 assesses new methodologies and the characterisation of future conditions. Chapters 17 – 20 assess responses to impacts through adaptation (17), the interrelationships between adaptation and mitigation (18), key vulnerabilities and risks (19) and, finally, perspectives on climate change and sustainability (20). This Technical Summary is intended to capture themost important scientific aspects of the full Working Group II Assessment. Reducing the information from 800 pages to 50 requires much condensing; consequently every statement in the Summary appears with its source in the Assessment, enabling the reader to pursue more detail. Sourcing information is provided in square brackets in the text (see Box TS.1). Uncertainty information is provided in parentheses (see Box TS.2 for definitions of uncertainty). Key terms are defined in Box TS.3.