Espionage is the practice of obtaining information about an organization or a society that is considered secret or confidential (spying) without the permission of the holder of the information. What differentiates espionage from other forms of intelligence work is that espionage involves obtaining the information by accessing the place where the information is stored or accessing the people who know the information and will divulge it through some kind of subterfuge.
Espionage is usually thought of as part of an institutional effort (i.e., governmental or corporate espionage). The term espionage is most readily associated with state spying on potential or actual enemies, primarily for military purposes, but this has been extended to spying involving corporations, known specifically as industrial espionage. Many nations routinely spy on both their enemies and allies, although they maintain a policy of not making comment on this. Black's Law Dictionary (1990) defines espionage as: "...gathering, transmitting, or losing...information related to the national defense."
A spy is an agent employed to obtain such secrets. The term intelligence officer is also used to describe a member of the armed forces, police officer or civilian intelligence agency who specialises in the gathering, fusion and analysis of information and intelligence in order to provide advice to their government or another organisation. Spymaster is a term often used in literature for the superior of a spy ring.
Incidents of espionage are well documented throughout history. The ancient writings of the Chinese military strategist Sun-Tzu and Indian prime minister Chanakya contain information on deception and subversion. The ancient Egyptians had a thoroughly developed system for the acquisition of intelligence, and the Hebrews used spies as well. More recently, they played a significant part in Elizabethan England (see Francis Walsingham). Feudal Japan often used ninja to gather intelligence. Many modern espionage methods were already then well established.
The Cold War involved intense espionage activity between the United States of America and its allies and the Soviet Union and the People's Republic of China and their allies, particularly related to nuclear weapons secrets. Recently, espionage agencies have targeted the illegal drug trade and those considered to be terrorists.
For three decades the United States has cultivated its best and brightest to pre-eminence in what is now known as the field of communication and control. As technology has advanced, the means and methods of espionage have advanced from Nixon era wire tapping, through Reagan era programs like ECHELON and Carnivore, to surveillance of all electronic transmissions including cell phone logs, voice mail, email, packet sniffing, trace routing and wireless transmissions.
However, the Soviet Union has been said to have had fielded the largest and most advanced spy networks during its time, infiltrating some of the most secure places on the planet, which has caused many scandals.
Since January of 2000, a long list of agencies have been data mining the world's stock exchanges; this program was formalized on October 26, 2001 in the form of the Patriot Act. This helps track the financing of people who might be laundering money from drug transactions. For a variety of reasons, including changes in technology, it has been necessary to do this without warrants and it is argued that the necessity makes it legal.
In order to gather political and economic information that might be of advantage to the United States, foreign communications are routinely subject to surveillance. In 2002, new programs of satellite surveillance and unmanned low level drones armed with missiles made it possible not only to perform surveillance in real time, but to respond with force.