The X-Men have more lives than a cat with a lazarus pit. They're probably the super-team in both Marvel or DC with the highest percentage of its membership having died at one point or another, and come back to life. And that's saying something.
Cyclical death patterns are part and parcel of comic book law, of course, with deaths of major and minor character used as a common technique to boost sales, but the X-Men suffer more than most. The mutants tend to fall victim to a high mortality rate due to their popularity, and the sheer amount of them makes it easy to pick off the occasional member. It happens with such alarming frequency that even the characters make light of it.
It's never anything to worry about though: characters who have been deep-sixed are constantly being brought back to life, as all it takes is one writer to come along who always wanted to write that specific character. They need only come up with a quick retcon using dodgy logic, and poof, it actually turns out that ticket they punched was for a return journey. But how good are the reasons behind their resurrections? Some returns haven't been so successful, but sometimes a death scene done right can be a really moving moment.
Here's a list of all those X-Men who have gone to worm food and back, and a comprehensive list of all the sketchy reasons why...