This guidance document is intended to help Food and Drug Administration (FDA) staff develop a request for additional information needed to make a decision on a medical device marketing application in accordance with the Least Burdensome Provisions of the Food, Drug, and Cosmetic Act (FD&C Act). Such an FDA request for additional information is known as a “deficiency.” In addition, this guidance describes suggested formats for FDA staff to communicate deficiencies, and for industry to use for responses to such requests, in order to make efficient use of industry and FDA’s time. This guidance includes examples of well-constructed deficiencies and industry responses to facilitate an efficient review process. This guidance also details supervisory review, major/minor deficiencies, additional considerations, and prioritization of deficiencies in FDA deficiency letters.