Espionage or spying involves a human being obtaining (i.e., using human intelligence HUMINT methods) information that is considered secret or confidential without the permission of the holder of the information. Espionage is inherently clandestine, as the legitimate holder of the information may change plans or take other countermeasures once it is known that the information is in unauthorized hands. See Clandestine HUMINT for the basic concepts of such information collection, and subordinate articles such as Clandestine HUMINT operational techniques and Clandestine HUMINT asset recruiting for discussions of the "tradecraft" used to collect this information.

Unlike other forms of intelligence collection disciplines, espionage usually involves accessing the place where the desired information is stored, or accessing the people who know the information and will divulge it through some kind of subterfuge. There are exceptions to physical meetings, such as the Oslo Report, or the insistence of Robert Hanssen in never meeting the people to whom he was selling information.

The US defines espionage towards itself as "The act of obtaining, delivering, transmitting, communicating, or receiving information about the national defense with an intent, or reason to believe, that the information may be used to the injury of the United States or to the advantage of any foreign nation. Espionage is a violation of 18 United States Code 792-798 and Article 106, Uniform Code of Military Justice[1]." The United States, like most nations, conducts espionage against other nations, under the control of the National Clandestine Service. Britain's espionage activities are controlled by the Secret Intelligence Service.

Espionage is usually part of an institutional effort (i.e., governmental or corporate espionage), and the term is most readily associated with state spying on potential or actual enemies, primarily for military purposes, but this has been extended to spying involving c