Although surveillance and espionage are two different things, they have in common that information has to be gathered that is not available otherwise.
In the case of intelligence agencies, many technologies have been developed during the Cold War, conceived for the defense and intelligence sectors. After the end of the Cold War these technologies have quickly spread to the law enforcement and private sectors.
At least some of these methods are not restricted to government organizations but could also be used by private organizations. In this section, I want to show which technologies are generally used.