The are many types of audio surveillance equipment, but they fall into two broad categories of remote and physical. Wiretaps and some other kinds of listening devices require that an eavesdropper be physically present at some point. Other forms of audio surveillance are done remotely, through "bugs" or by subverting existing technologies that use wireless.
Wiretapping is one of the most well-known versions of audio surveillance equipment. Before wireless phone technology was invented, it involved creating an electrical connection between the subject's phone wire and the surveillance team's so that a conversation could be listened to in real time. This connection was called a "tap".
With the rise of wireless technology, all telephone conversations are tracked by computer. Telephone companies can have their computers perform a "digital switch", where a conversation's audio is copied to a second line. Though it is performed remotely, this kind of audio surveillance is still referred to as "wiretapping" and is almost impossible to trace.
Enhanced listening devices are a more hands-on form of audio surveillance equipment. A more advanced version of holding a drinking glass up to the wall, these microphones are designed to hear through walls, and an eavesdropper must physically be in the same building as the subject for them to be placed. Many of these devices primarily consist of a powerful microphone and headphones.
A bug is another common form of audio surveillance equipment. Bugs are tiny microphones equipped to send out information through radio waves. They can be hidden almost everywhere. In 1950, a bug was placed in a wooden replica of the Great Seal, used by the United States government to seal documents, and given in friendship to the U.S. Ambassador by Russia. It wasn't discovered until a decade later.
Miniature recorders are also a readily available form of audio surveillance equipment. Like bugs, they can be hidden in innocent objects such as pens or lapel buttons. Such devices can be purchased over the Internet, but laws vary as to how legal it is to own one.
Today, surveillance experts are finding ways to turn hand-held devices into audio surveillance equipment, rather than smuggling in bugs or training microphones on conversations. The Federal Bureau of Investigation (FBI) in the U.S. has the technology to remotely turn on a cell phone in a subject's pocket. Agents are then able to listen in on conversations being held in the vicinity. This method is called The Roving Bug.
In 2003, the Internal Revenue Service (IRS) in the U.S. discovered a way to listen in to a conversation through a car's emergency tracking system. The U.S. Court of Appeals for the Ninth Circuit prohibited this method of surveillance on the grounds that it disabled all of the car's security systems.