This paper describes the sources of emissions of ammonia gas and their impact on the wider environment, on the cycling and utilisation of nitrogen in soils and on the health of humans and farm livestock. It summarises the research that has been undertaken to develop and evaluate methods for abating ammonia emissions and to identify the most cost-effective and widely applicable. EU Member States are signatories to international agreements to limit ammonia emissions. Under the UNECE Gothenburg Protocol and the EU National Emissions Ceilings Directive countries have agreed to national limits (or "ceilings") on ammonia emission to be achieved by the year 2010. Reduction in emissions from large pig and poultry farms may also be required under the EU Integrated Pollution Prevention and Control Directive. Ammonia emissions can be reduced by the introduction of technical measures on livestock farms. Although a wide range of such measures has been addressed experimentally, relatively few have proved to be cost-effective, practical and widely applicable on commercial farms. These include livestock feeding strategies to reduce nitrogen excretion (and hence the potential for ammonia emission), new designs for livestock housing and, most importantly, methods for reducing emissions from the storage and spreading of manures.