It is widely assumed that the more information surveillance apparatuses can collect about an individual, the less risk she poses. In this article, we examine how gender figures into and potentially disrupts the link between identity and security. Our analysis centers on one very particular event: the confusion that erupts at the airport when US Transportation Security Administration agents perceive a conflict between the gender marked on one's papers, the image of one's body produced by a machine, and/or an individual's perceived gender presentation. Gender has been so deeply naturalized—as immutable, as easily apprehended, and as existing before and outside of political arrangements—for so long that its installation in identity verification practices largely goes unthought. In what follows, we describe how the two TSA programs, "Secure Flight" and "Advanced Imaging Technology," operationalize gender differently. We examine what happens when different sources of knowledge about gender clash within the security assemblage of the airport. As part of state security apparatuses' unceasing quest for more and better information, both programs securitize gender. They also reveal the impossibility of predicting with certainty that something about a person, even something thought to be sourced/lodged in the body such as gender, will stay the same over time. We argue, however, that the effects of gender's unreliability as a measure of identity do not constitute a problem for the TSA but rather for the transgender individuals whose narratives, documents, and bodies reveal the category's mutability. We conclude by suggesting that the securitization of gender at the airport is best understood as an assemblage.