This 21st issue of Spectrum features an address by the President of Burkina Faso, Blaise Compaoré, who became the first head of state to address the CTBTO Preparatory Commission at its 40th Session on 13 June 2013. Compaoré shares his vision for peace and international security in a world without nuclear weapons, calling on those remaining States that have not yet signed or ratified the CTBT to do so in order that the Treaty can become legally binding. He also describes the usefulness of CTBTO monitoring data which “are of great importance in defining effective responses to natural risks and disasters.” As Co-Presidents of the Conference on Facilitating the Entry into Force of the CTBT at the UN Headquarters in New York on 27 September, Hungary’s Minister for Foreign Affairs, János Martonyi, and Indonesia’s Minister for Foreign Affairs, Marty M. Natalegawa, explain why the CTBT’s entry into force is of paramount importance. Outlining their priorities in promoting this objective, Martonyi calls on the remaining Annex 2 States, especially the United States, to ratify. Appealing to the United States as a NATO partner, he states that “ratification by the United States is in no way detrimental to NATO’s nuclear deterrent, but that it would, on the contrary, enhance global security.” Expressing his concern over the nuclear tests announced by North Korea in 2006, 2009 and 2013 which have exacerbated tensions in the region, Natalegawa says: “Such nuclear tests highlight the urgent need for the CTBT’s entry into force “ Iceland’s Minister for Foreign Affairs, Gunnar Bragi Sveinsson, maintains: ‘It is abundantly clear that all non-proliferation efforts are critical in tackling the threat of terrorists acquiring nuclear weapons. A fully ratified and implemented CTBT is an indispensable building block for these efforts.” He also highlights the potential use of CTBT monitoring data in helping to mitigate the effects of natural or man-made disasters, particularly in terms of monitoring volcanic eruptions, which he explains is of great interest to Iceland. This issue also features excerpts of keynote addresses made at the CTBT: Science and Technology 2013 Conference in Vienna, Austria, in June 2013. These include: former head of the International Atomic Energy Agency, Hans Blix; former U.S. Under Secretary of State for Arms Control and International Security Affairs, Ellen Tauscher; former Director of the Los Alamos National Laboratory, Siegfried Hecker; and Director for Strategic Affairs in the French Ministry of Defence, Michel Miraillet. The keynote speakers present a range of arguments as to why countries that have not yet ratified the CTBT should delay no longer. Miaki Ishii from Harvard University describes how the CTBTO’s seismic stations “serve unexpectedly well as powerful telescopes to view inside the Earth.” Detailed knowledge of the internal structure of the Earth is essential, she explains, for unravelling its dynamics and history. An article on ‘Joining forces to reduce radioxenon emissions’ highlights some of the recent collaboration between the CTBTO and radioisotope producers such as the Belgian-based Institute for Radioelements to achieve this goal. With preparations for the next Integrated Field Exercise in Jordan in 2014 well underway, Spectrum 21 features a photo story depicting ‘Build-Up Exercise III’ which took place in Hungary from 26 May to 7 June 2013. This exercise will simulate an on-site inspection almost in its entirety. Also on this theme, an article by the CTBTO’s Aled Rowlands describes the role of airborne imagery in an on-site inspection.