This publication presents the proceedings of the latest IAEA symposium on isotopes in hydrology, marine ecosystems and climate change studies. At the symposium, five major topics were addressed through invited talks and oral presentations. These five sessions covered: the role of isotopes in understanding and modelling climate change, marine ecosystems and the water cycle; carbon dioxide sequestration and related aspects of the carbon cycle, such as ocean acidification; isotopes and radionuclides in the marine environment; groundwater assessments for large aquifers; and analytical methods and instrumentation for the application of isotopes in environmental, climate and hydrological studies. Leading scientists in the field of climate change and hydrology, as well as representatives from climate change and environmental bodies and organizations, exchanged their views and experience.