The aim of my thesis is to prove the hypothesis that the National Security Agency (NSA) has illegally carried out mass surveillance of US and other nationalities. In order to do so, I will present the history of the NSA (including universal and American historical events in which the agency has played a role, such as the Vietnam War or the American Civil Rights Movement), using sources from historians and experts on the history of the intelligence agency. I will also present the currently known structure of the intelligence agency. I will discuss how the intelligence agency's operations have evolved in the post-9/11 period, and finally, I will present how the NSA has carried out illegal surveillance, based on the accounts and evidence of Edward Snowden.