This book constitutes the proceedings from a symposium titled Separations for the Nuclear Fuel Cycle in the 21st Century which was held in March 2004. This symposium focused on assessing the current state-of-the art in nuclear separations science and technology, and on identifying R&D directions required to enable nuclear separations to meet 21st Century demands for waste minimization, environment protection, safety, and security. Several recent developments have elevated the importance of this topic including: the potential effects of global warming caused by carbon dioxide emissions from burning of fossil fuels; rising demand for the electric power that is needed to raise the global standard of living; geopolitical issues such as those related to nuclear proliferation and nuclear terrorism. Although expansion of fission-based nuclear power would help reduce the potential for global warming, future development of the nuclear fuel cycle must be weighed against the issues of proliferation and management of irradiated fuel. Separations will play a central role in addressing these issues. The subject matter of this book is organized into five topical areas. These are: 1) current trends and direction to the future, 2) aqueous processing, 3) emerging separations systems, 4) actinide/lanthanide separations, and 5) solution-solid interactions. This collection of papers provides a snapshot of the current state of nuclear separations chemistry and can be used to help guide future directions in this critical technological field.