The dome at the top of Saint Peter's Basilica can be seen from most places in Rome. The design of the cupola, like the basilica itself, went on for over a hundred years and passed through many popes' and architects' hands. However, the final design has mostly been credited to Michelangelo. It is a magnificent part of the Vatican's skyline, as well as the Eternal City's and a must-climb for any able bodied tourist. Honestly, it is spectacular in every possible way, from every angle - and, true to form, it offers quite the spectacle of a trip up.
The very top is just under 500 feet high. So, as you can imagine, it's quite a trek. I tried to count the steps as we walked. I'm pretty sure it is 239 up to the drum of the cupola, where you can stand and gaze up at the beautiful interior. It's also pretty amazing to look down onto the floor of the basilica and realize just how much higher up you are than the little ant people below. Most trekkers take the elevator to this point, saving themselves that generally uninteresting first chunk of stairway. However, if they want to go out and stand on the very top of the dome, out around the exterior, they've gotta do the rest sans lift. 
And that's really where the fun part begins. The next 332 steps are a steep, narrow, spiral. At around step 356 (from the bottom - 117 from the elevator) you've begun to scale the dome itself. The outside shell of the dome is raised a little further out from the inner ceiling and you are walking right in between the two. It was a pretty crazy optical illusion, standing straight up and looking forward to see the person in front of me at a 45 degree angle. Our bodies curved against the sphere as we walked in a circle, higher and higher. At step 498 (259 from the lift) it becomes so steep and so narrow that a rope hangs down the center for you to hold on to.
Then, you emerge! The fresh air alone is enough for people to cherish this moment. Throughout the trip up, there are small slivers of window, but not much else. However, when you make your way to the edge and realize that you are indeed at the top of the largest church in the world, looking out over Saint Peter's Square and the rest of Rome, it is a reward like no other. Merlin used a photo from up top in his Vatican Gardens post and will also in his look at the square, so I won't bother with more here. Instead, enjoy the view of people enjoying the view.
On the way down, you get to access the basilica's roof terrace. It's a gorgeous place to spend some time and there's absolutely no rush to get going on down. I've been up three times and it's never felt full. There's a bathroom (!) that rarely has a line and a potable water fountain. Large seagulls perch and pose for pictures on the decorative cupolas scattered about. I thought it was just the coolest place before I even walked to the front and realized exactly where I was.
These are the thirteen statues that line the top of Saint Peter's. Logically, I knew they would have to be enormous to be seen from so far below, but it was still pretty crazy to encounter their magnitude. Jesus is at the center, flanked by eleven of his twelve apostles (Peter's statue in the series is down in the square) and John the Baptist. If you look closely, you can see a little rectangle at the base of the closest statue, which read "S. Matthias." Matthias took Judas' place in 'the twelve' after the whole betrayal/suicide incident. Judas didn't make the final statue cut.
On the roof you will also find a Vatican post office, a souvenir shop run by nuns and a cafe that serves surprisingly low-priced refreshments. It really feels like some secret village at the top of the world, blissfully above and away from the madding crowd.
Before heading back down, I refilled the plastic cup my cafe-served water came in at one of the fountains. The walk down was slightly shorter than it was up (only 400 steps) and much simpler to navigate. We were a little disappointed that the dome closes at 5pm, far too early to catch a sunset - but long daylight hours aren't really something we're going to complain about.Just a word of advice: signs at the cupola ticket office repeat over and over that those who are unfit or elderly should be advised that there are 300+ steps to scale even with the elevator. I don't think that many people understand that those stairs will be continuous, that there won't be many opportunities to rest - if any - and that it is very claustrophobic. Also, they are probably not allowed to comment on people's width, but it is simply to narrow for larger people (and too low for extremely tall folks). So, please, be advised.