The Under Secretary of Defense for Acquisition and Sustainment, or USD (A&S), is the Principal Staff Assistant (PSA) and advisor to the Secretary of Defense for all matters relating to acquisition and sustainment in the Department of Defense. This includes the DoD Acquisition System; system design and development; production; logistics and distribution; installation maintenance, management, and resilience; military construction; procurement of goods and services; material readiness; maintenance; environment and energy resilience ; utilities; business management modernization; International Armaments Cooperation, Cooperative Acquisition and International Agreements, Promoting exportability of military components to allies and partners; nuclear, chemical and biological defense programs; and nuclear command, control, and communications.

The Under Secretary of Defense for Research and Engineering, abbreviated USD (R&E), is a senior official of the United States Department of Defense. The USD (R&E) is charged with the development and oversight of technology strategy for the DoD. The post has at various times had the titles Assistant Secretary of Defense for Research and Engineering, or Director of Defense Research and Engineering (DDR&E). The latter title has itself historically varied between the rank of under secretary and that of assistant secretary.