National Institute for Public Policy, Senior Scholar, 2005-Present
U.S. Special Envoy for Nuclear Nonproliferation, Prior to July 2007
Under Secretary of State for Arms Control and International Security, 2005-2007
Special Assistant to the President and Senior Director for Proliferation Strategy, Counterproliferation and Homeland Defense, National Security Council, 2001-2004
National Defense University, Professor of National Security Studies and Director/Founder of the Center for Counterproliferation Research, 1994-2001
U.S. Commissioner to the Standing Consultative Commission and Ambassador to the U.S.-Russian Commission on Nuclear Testing
Principal Deputy Assistant Secretary of Defense for International Security Policy, Deputy Assistant Secretary of Defense for Nuclear Forces and Arms Control Policy, Nuclear Policy/Planning Officer at U.S. Mission to NATO.