Global effects to overcome the growing challenge of freshwater availability have been at the forefront of the world development agenda for nearly three decades. For developing policies towards sustainable management of freshwater resources, an improved understanding of the Earth’s water cycle bas been widely recognized as one of the key elements of scientific information.The IAEA has played a crucial role in promoting and expanding the field of isotope hydrology. Starting in 1963, the IAEA’s quadrennial symposia on isotope hydrology have played a central role in developing this scientific discipline. This publication contains 174 extended abstracts of papers presented during 11 technical sessions of the 11th symposium in the series that was convened during 19–23 May 2003 in Vienna. Nearly 275 participants from 69 countries participated in the symposium to discuss the past, present and future of isotope applications in hydrology and climate research.