This page is being written by someone who eats late, always. Follow these guidelines and you'll be eating well... late into the night:

24-hour diners
The staple of the late-nighter. Pittsburgh boasts two notable ones, Tom's Diner at 1715 E Carson St and Ritter's Diner at 5221 Baum Blvd. You'll get all the usual diner fare with a disgruntled wait staff. Simply can't be beat. A few Eat N Park locations around town are 24-hour as well. Check their website http://www.eatnpark.com for locations. The difference here is that you might actually get a pleasant waitstaff. And that takes away from the experience altogether!

Exclusively Pittsburgh
Primanti Brothers ( http://www.primantibros.com/) is a Pittsburgh icon. If you don't know, I'm not going to explain here. Check the website and read for yourself. While only the Strip District location is open 24 hours, the other locations stay open well into the wee-hours of the morning. Take note here, the sandwiches taste much better after a night of a little too many drinks. In fact, I would postulate that is what they were made for!

Pizza
The best late night pizza you can find depends largely on what neighborhood you find yourself in. Follow along and pay attention:
Scenario 1: You've been enjoying shaking your rump at one of the many dance clubs in the Strip, and now it's 2AM. Where to go for pizza? Easy answer, Bella Norte at 1914 Penn Ave. Huge, thick pieces that are made fresh, no frozen stuff back there. This is sure to stave off any potential hangovers.
Scenario 2: Bar crawling your way down Carson Street on the South Side, or even just walking along and people-watching, all of a sudden the craving for pizza hits you. You can't go wrong at Pizza Vesuvio at 1417 E Carson St. A tiny place that you can easily miss if you're not paying attention. But this place serves it up real NYC style and for that matter, true Italian style. Thin crust, perfect cheese and toppings, I'm getting hungry just thinking about it. There's very little seating here, it's mainly a to go place only.

Cheesesteaks - Philly-style
Although I haven't confirmed it, I'm willing to bet the owner of this place hails from about 300 miles east of Pittsburgh, namely the City of Brotherly Love. Because this is the only cheesesteak in town where you can order cheese whiz on your steak and it doesn't come with lettuce and tomato without you specifically asking for it (for the unknowing, that is a cheesesteak HOAGIE). You'll swear you're in South Philly after scarfing down a cheesesteak from South Side Steaks at 2000 E Carson Street.

And if you're just plain crazy
There is a famous, or infamous, establishment in the heart of Oakland known as the Original Hotdog Shop, or just  "The O", or in some circles, "The Dirty O". You take your life into your own hands when heading here for a late night bite. Don't believe me? You pay an extra 10% to all menu items after 10PM as a "security charge". No kidding. If you think you're a big shot and want to try it out anyway, you won't be sorry at least as far as the food goes. You can get most anything your heart desires here, pizza, sandwiches, burgers, and far-and-away the best fries in Pittsburgh. They claim to spill more fries on the floor here than McDonalds serves. You might actually end up believing that if you order an extra-large fry. I won't give anything away here, try it for yourself.