Police are using it as a powerful tool; they say to track down the bad guys.
But could it also be tracking down us good guys in the process?
Last May, Tempe police were on the hunt for the suspects who carried out a double murder near Kyrene and Elliot roads. The youngest victim, barely old enough to drive.
One week later, four people were arrested with the help of a device known as a "Stingray."
"Who knows what could have happened if we did not have that technology," explains Tempe Police Sgt. Ronald Elcock.
Tempe police have had a Stingray for the past nine years.
"It's been huge for us," Sgt. Elcock said.
So huge, in fact, the department is now asking the City of Tempe for $750,000 to purchase a new, updated Stingray system.
"With locating suspects, you can't put a price tag on getting information that's able to get someone off the streets."
But the device doesn't come without controversy. The ACLU has come out against Stingrays and other "cell site simulators," calling them "invasive."
The group worried the devices could actually be harvesting personal information of innocent people while police are trying to track down the bad guys.
But Tempe police says, first of all, they're only scanning for cell phone serial numbers, nothing else.
"We don't use it to capture conversations. We don't use it to capture conversations at all. We don't use it to capture text messages."
Police must also obtain a search warrant from a judge before they can deploy the device.
Any information that's collected must be deleted within 60 days per state law. Tempe police say they usually delete information even faster than that, usually between 24 to 72 hours.
Tempe police are not the only law enforcement agency with a Stingray. Phoenix, Scottsdale and the Maricopa County Sheriff's Office also have them.