This publication is one of the series of IAEA publications on the environmental behaviour of naturally occurring radionuclides. It outlines uranium behaviour in different environments, as well as its transfer to, and metabolism in, humans. The publication also provides concepts, models and data required for the assessment of the impacts of uranium on non-human biota. Assessing the environmental and health effects of uranium poses specific challenges because of the combination of different types of hazard and potential exposures. Therefore, both the radiotoxicity and chemical toxicity of uranium are considered in this publication.