Yes / In the discussions that have taken place over the past six years since the establishment by the Third Review Conference1 in 1991 of the Ad Hoc Group of Governmental Experts,known as VEREX, to identify and examine potential verification measures from a scientific and technical viewpoint, there has been an increasing debate about the role of non-challenge visits in a regime for a strengthened Biological and Toxin Weapons Convention (BTWC). The arguments as to why on-site investigations are an essential and central element to such a strengthened regime were addressed in the Briefing Paper issued2 in July 1997. In this Briefing Paper, the necessity for non-challenge visits is addressed drawing upon the previous VEREX, Ad Hoc Group (AHG), Chemical Weapons Convention (CWC) and United Nations Special Commission (UNSCOM) experience. The advantages and disadvantages of a regime containing non-challenge visits are considered and the conclusion is reached that the advantages far outweigh the disadvantages and that non-challenge visits are an important element which could contribute significantly to the effectiveness of a future legally binding instrument to strengthen the BTWC.