An agreement on Conventional Armed Forces in Europe (CFE) may place numerical and geographical limits on more than 140,000 treaty-limited items (TLIs)1 in 21 countries. Monitoring limits on such huge numbers of TLIs would be extremely difficult, as well as expensive and intrusive, with human inspectors alone. This chapter examines a promising way to effectively monitor limits while reducing cost and intrusiveness: the tagging of TLIs. The use of tags transforms a numerical limit into a ban on untagged items. The result is that many of the verification advantages of a complete ban can be retained for a numerical limit. Tagging works by certifying that every TLI observed is one of those permitted under a numerical limit. A tagging system would involve the manufacture of a number of tags equal to the number of TLI, which would then be affixed to an essential part of each allowed TLI. If even one untagged TLI were ever seen—during on-site inspections (OSI), by national technical means (NTM), or even by nationals of the inspected party loyal to the treaty regime—then there would be prima facie evidence of a treaty violation. If properly designed, tags could also identify a TLI as belonging to a particular nation or as normally deployed in a particular region, which would make it easier to verify CFE sub-limits on national and regional deployments.