This testimony discusses our past work examining the Department of Homeland Security's (DHS) progress and efforts in planning, developing, and deploying its global nuclear detection architecture (GNDA). The overall mission of the GNDA is to use an integrated system of radiation detection equipment and interdiction activities to combat nuclear smuggling in foreign countries, at the U.S. border, and inside the United States. Terrorists smuggling nuclear or radiological material into the United States could use these materials to make an improvised nuclear device or a radiological dispersal device (also called a "dirty bomb"). The detonation of a nuclear device in an urban setting could cause hundreds of thousands of deaths and devastate buildings and physical infrastructure for miles. While not as damaging, a radiological dispersal device could nonetheless cause hundreds of millions of dollars in socioeconomic costs as a large part of a city would have to be evacuated--and possibly remain inaccessible--until an extensive radiological decontamination effort was completed. Accordingly, the GNDA remains our country's principal strategy in protecting the homeland from the consequences of nuclear terrorism. The GNDA is a multi-departmental effort coordinated by DHS's Domestic Nuclear Detection Office (DNDO). DNDO is also responsible for developing, acquiring, and deploying radiation detection equipment to support the efforts of DHS and other federal agencies. Federal efforts to combat nuclear smuggling have largely focused on established ports of entry, such as seaports and land border crossings. However, DNDO has also been examining nuclear detection strategies along other potential pathways and has identified several gaps in the GNDA, including (1) land border areas between ports of entry into the United States; (2) international general aviation; and (3) small maritime craft, such as recreational boats and commercial fishing vessels. Developing strategies, technologies, and resources to address these gaps remains one of the key challenges in deploying the GNDA. Some progress has been made, but DHS and other federal agencies have yet to fully address gaps in the global nuclear detection architecture. Specifically, this testimony discusses DHS's efforts to (1) address our prior recommendations to develop a strategic plan for the GNDA, including developing strategies to prevent smuggling of nuclear or radiological materials via the critical gaps DNDO identified, (2) complete the deployment of radiation detection equipment to scan all cargo and conveyances entering the United States at ports of entry, and (3) develop new technologies to detect nuclear or radioactive materials. This testimony is based on our prior work on U.S. government efforts to detect and prevent the smuggling of nuclear and radiological materials issued from October 2002 through September 2010. We updated this information in July 2011 to reflect DHS's efforts to address our prior recommendations by meeting with DNDO officials and reviewing recent DNDO documents, such as the 2010 GNDA Strategic Plan and the 2011 GNDA Joint Annual Interagency Review.