Spies working for states fall into two categories: intelligence officers and agents.
Confusion often arises between what is meant by an officer and an agent. Other countries use the same terminology in different ways. In the United States, for instance, an agent is a member of an intelligence or security agency such as the FBI or CIA. Such agencies call a covert human intelligence source an "informant" rather than an "agent."
Intelligence officers seek to gather covert intelligence directly and to recruit agents to obtain intelligence on their behalf. Foreign intelligence services typically seek to establish networks of agents whom they can use over a sustained period of time, so that they can obtain a reliable flow of information.
The methods used by intelligence officers vary widely, and are often limited only by their ingenuity. They will often take advantage of the latest technology, using it to eavesdrop, tap telephone calls and communicate secretly. However, the human relationship between an intelligence officer and his or her agents remains a key element of espionage.
Agents operate by exploiting trusted relationships and positions to obtain sensitive information. They may also look for vulnerabilities in handling secrets. If they are insiders, they may be aware of flaws in their organisation's security that they can exploit.