They've been fixtures on commercial flights around the world for decades.
And in the inquiry into what caused Metrojet Flight 9268 to crash, investigators hope they'll provide crucial evidence.
Here's a look at what we know so far about the Russian plane's cockpit voice recorder and flight data recorder, the so-called black boxes, which were found at the crash site on Saturday:
Where are they now?
The flight crashed in Egypt's Sinai Peninsula on Saturday, killing all 224 people on board. After the crash, Egyptian officials said the two black boxes had been found and were being transported to Cairo for analysis.
The Egyptian Civil Aviation Ministry released a photo of the boxes on Monday after officials from Egypt and Russia examined them.
Officials have said the boxes are in good condition. Russian Transport Minister Maxim Sokolov said they had sustained minor damage, according to state media.
What can investigators learn from them?
The cockpit voice recorder captures sounds on the flight deck that can include conversations between pilots, warning alarms from the aircraft and background noise. By listening to the ambient sounds in a cockpit before a crash, experts can determine if a stall took place and the speed at which the plane was traveling.
The flight data recorder gathers 25 hours of technical data from the airplane's sensors, recording several thousand discrete pieces of information. Among the details investigators could uncover: Information about the plane's air speed, altitude, engine performance and wing positions.
A Metrojet official said Monday that the black boxes hadn't been read or decoded yet.
"The likelihood," CNN aviation correspondent Richard Quest said, "is that the black boxes will tell us the cause of why this plane suddenly decelerated very fast."
But black boxes aren't perfect. In several cases — like the 1996 crash of TWA Flight 800 or the crash of American Airlines Flight 77 on September 11, 2001 — authorities had hoped to find clues in the black boxes, only to discover that the data inside had been damaged or the recordings had stopped suddenly.
Who will be reading and interpreting what's inside?
Because the plane crashed in Egypt, authorities there are heading the investigation.
Egyptian President Abdel Fattah al-Sisi has promised Russian President Vladimir Putin that he will allow "the broadest possible participation of Russian experts in the investigation," according to the Kremlin.
Putin has also ordered Russian Prime Minister Dmitry Medvedev to open an investigation, the Kremlin said.
Aviation investigators from France and Germany, the countries where the plane was manufactured, are also taking part.
The aircraft's engines were manufactured in the United States. If the plane's engines become a focus of the investigation, the US National Transportation Safety Board will likely dispatch a team to Egypt as well, a US official with knowledge of the investigation said.
