In Marvel's upcoming Phoenix: Resurrection, the adult Jean Grey will finally be coming back from the dead. Over the years, Jean Grey dying and coming back to life has become a bit of a joke, but at the same time, her deaths (and returns) have been generally overstated to a large degree.
While yes, any death and return to life should be considered significant, when it comes to comic books, such things are surprisingly common. Therefore, it was only in the 21st Century that Jean Grey's deaths and resurrections really got out of hand. Here, we'll detail her comic book history of deaths and returns, while noting that we're not counting stories where the entire world briefly is destroyed (like the famous Sise-Neg storyline in Doctor Strange's comic book series in the 1970s), but stories where she is specifically noted as having died.
The first time that Jean Grey seemingly died was during the classic conclusion of X-Men #100 (by Chris Claremont and Dave Cockrum). The X-Men had just escaped from a space station on a special space plane piloted by Dr. Peter Corbeau, but during the travel to the station (where the team had traveled to rescue three of their kidnapped team members - Wolverine, Banshee and Marvel Girl, who was actually not technically a member of the X-Men at the time and was only with the X-Men for Christmas Eve because Jean was dating the X-Men's team leader, Cyclops) the ship's shields had been damaged in the area of the plane where you would manually pilot the plane. Sadly, the automatic landing system was also damaged. There were some severe solar flares going on, so whoever was stuck flying the plane without the shields was going to die from solar radiation, but someone had to pilot the ship or else it would never land back on Earth. It was a suicide mission and Jean Grey volunteered - using her telepathic powers to take the information from Dr. Corbeau's brain on how to land the ship and then hopefully would use her telekinetic powers to shield herself. It did not go so well (well, the landing part actually went okay, but not so much the "shielding herself" part)...
The next issue (by Claremont, Cockrum and Frank Chiaramonte), the ship crash landed into a bay outside of New York City (okay, so even the landing part did not go especially well) and while the X-Men all collected themselves in the water, they were shocked to see that Jean Grey might have died, but she had now been reborn as the Phoenix, named after the mythical bird who comes back to life after it burns to death!
So Jean/Phoenix rejoined the X-Men and was now the team's most powerful member. However, after she was split off from the rest of the team (Jean believed that they had died in an explosion after a fight with Magneto at his Antarctic base and they thought that she had died in the same explosion), she ended up becoming targeted by the evil Jason Wyngarde, the longtime X-Men foe known as Mastermind. Mastermind used his powers of mental manipulation to try to woo Jean Grey to his side so that he could use her to become the Black Queen of the Hellfire Club, while he would become the Black King. However, as it turned out, messing with the mind of someone who could take on one of Galactus' heralds easily was not a good idea and Jean snapped, turning into Dark Phoenix. As Dark Phoenix, she headed off into outer space and destroyed a Shi'ar space ship and an entire planet of sentient beings. The Shi'ar did not take kindly to that, so they decided that they had to kill her.