Occupational hygiene, in its broadest sense, is the recognition, evaluation and control of health hazards arising from work – a trained hygienist needs to understand a range of subjects from health and safety legislation through to toxicology, hazardous substances, physical agents, exposure assessment and exposure control strategies. To some extent, occupational hygiene is relevant in all workplaces although, in practice, it is most applicable to environments in which significant health risks exist. These can range from call centers with noise hazards to chemical sites handling bulk quantities of very toxic substances. The legislation and practical measures for managing human exposure to chemicals will vary from one national jurisdiction to another but, in the United Kingdom, they are implemented in the Control of Substances Hazardous to Health (COSHH) Regulations (http://www.hse.gov.uk/coshh/) produced by the Health and Safety Executive (HSE), which recommend using valid and suitable occupational hygiene techniques to estimate the amount of employee exposure to substances hazardous to health. Exposure monitoring has played a major role in the development and evolution of UK occupational hygiene and is a key element in the ‘evaluation’ stage of the recognition, evaluation and control philosophy. Monitoring is often invaluable when making an initial assessment of a work environment, providing quantitative evidence to allow development of robust exposure control strategies. Subsequent repeat monitoring may be required to demonstrate that adequate exposure control is being achieved and maintained over time.