Abstract The objective of an environmental monitoring and sampling programme is to detect undeclared nuclear activities. Although the ability to detect any type of nuclear activity would be important, the first priority is to detect the production of separated plutonium or HEU, i.e., material that is in, or can be processed relatively easily into, a form in which it can be incorporated directly into a nuclear weapon. Thus the ability to detect undeclared reprocessing, or conversion of the plutonium nitrate product to Pu metal, enrichment of uranium to HEU or conversion of HEU product to HEU metal is the key requirement. From the IAEA safeguards point of view, there are two options for a proliferation in carrying out undeclared activities: in a declared nuclear site that is subject to safeguards inspections; or elsewhere in the State. Environmental monitoring and sampling (EMS) can be used, in principle, to detect both categories, via short- or long-range monitoring. In this chapter we consider the environmental signatures likely to arise as a result of nuclear fuel reprocessing on a relatively small scale as part of a covert nuclear weapons programme. The technology required to reprocess nuclear fuel and extract plutonium is much simpler than that required to enrich uranium. Signatures of undeclared activities could be signatures of an undeclared reactor, undeclared HEU production or signatures from reprocessing of Pu. All three scenarios are discussed in some details. One of the important lessons learned from the preparation for the Chemical Weapons Convention (CWC) was the necessity of at least two different spectrometric techniques to absolutely determine compound identification. Additionally, expert interpretation of the results is necessary.