An attack on a building or location using non conventional weapons will almost certainly lead to a requirement to undertake some form of decontamination of that site. This simple observation immediately raises the question "what decontamination standards and techniques should be used?." These questions are important and they have been the subject of much technical research - often under the banner of "how clean is clean" and "what is reasonable in terms of clean-up costs and decontamination measures." This booklet seeks to address some of these issues.