A cell phone jammer is a device that emits signals in the same frequency range that cell phones use, effectively blocking their transmissions by creating strong interference. Someone using a cell phone within the range of a jammer will lose signal, but have no way of knowing a jammer was the reason. The phone will simply indicate poor reception strength.

With the ubiquitous use of cell phones, a backlash has occurred. While some people practice cell phone etiquette, many others noisily discuss their private, professional or mundane business in public areas, forcing everyone nearby to listen. On trains, subways, buses, in the grocery market, shopping mall and café, people are aggravating fellow citizens with their non-stop chit-chat. This has caused some people to take matters into their own hands. With a cell phone jammer in purse or pocket, jabbermouths can be turned off with the flip of a switch — and they won't be able to reconnect as long as the jammer is activated unless they move far enough away from the source.

It seems a tidy solution, however, there's a problem. Cell phone jammers are illegal in most countries — except to military, law enforcement and certain governmental agencies.

In the U.S. the FCC (Federal Communications Commission) makes certain frequencies available to broadcasters for public use. When an end-user pays to use that spectrum, jamming the signal is paramount to 'property theft.' The FCC is also concerned about potential "leakage" — of jammers interfering with frequencies outside the range of cell phones, like garage door openers or medical equipment; and it's worth noting that over 100,000 emergency calls are made each day from cell phones. Anyone caught manufacturing, selling, owning, or using a jammer in the U.S. is punishable by an $11,000 fine and up to a year in prison for each offense.

But the stiff penalty hasn't stopped proliferation of the devices, perhaps because the FCC has not held anyone accountable. According to one interview with Richard Welch, associate chief of the FCC's Enforcement Bureau, no actions have been taken by the FCC because "nobody has complained." This isn't surprising considering people can't tell the difference between being jammed and simply having poor signal strength which comes and goes with the best of phones even under normal circumstances.

Cell phone jammers are available in different styles and sizes from personal hand-held models that look like cell phones themselves, to units that resemble routers with multiple antennas, to even larger briefcase-style jammers. While personal jammers create a bubble of anywhere from 30 — 100 feet (9 - 30 meters) depending on the model, more powerful devices can create "dead space" of up to a mile (1.6 km) in radius. This can be useful around a Presidential motorcade, for example, to keep terrorists from detonating a bomb from miles away or even from outside the country. By wiring a cell phone to explosives the device can be triggered by simply placing a call to the phone, as was done in May 2002 by Palestinian militants in Tel Aviv when they targeted an Israeli fuel depot by rigging one of its fuel trucks.

Law enforcement also uses cell phone jammers in hostage situations to keep the suspect isolated, and in South America, banks use the devices to prevent robbers from tipping off outside accomplices to departing customers leaving with large withdrawals.

Proprietors of many kinds of businesses would like to use cell phone jammers. Restaurant owners and theater houses are just two examples of places that regularly receive complaints from patrons over cell phone abuse. Short of providing expensive metal shielding in the construction of the buildings to block cell phone signals, (which is legal), it's understandable that placing an inexpensive device in the back office to surreptitiously block cell phone usage in the establishment might be tempting. Hospitals would also like to jam cell phones which can interfere with medical equipment. Churches, libraries, courthouses and business owners that want to boost employee productivity are all examples of potential customers of cell jamming technologies.

Personal cell phone jammers start at about $250 (U.S.D.) and are widely available online despite their illegal status in most countries. The top manufacturers reportedly sell primarily to military and law enforcement but will sell the devices to anyone with the disclaimer that it is up to the buyer to make sure the device is legal in his or her country and that the buyer assumes all legal responsibility for buying, owning, or using the device.

The cell phone industry opposes the use of jammers and many have invested money in education towards cell phone etiquette as an alternative answer to the growing problem of discourteous cell phone users.