This is probably the most-asked question on the Philadelphia forum, so here are the most recommended places (in no particular order)...

Jim's, 4th & South Streets, convenient to the historic area, Society Hill, The Italian Market, and, well, South Street. You'll know you're close when you smell the fried onions, so just follow your nose.

Campo's, 2nd & Market Streets. Most convenient to the historic area, Penn's Landing, and Old City.

Tony Luke's, Front & Oregon. You need a car to get here, but it's only about a 10 minute drive from downtown. Convenient to the stadiums. The roast pork sandwhich tends to get better reviews than the cheesesteak, but you can't go wrong. You will also get more of a "South Philly" experience here than at the two above.

Rick's, Reading Terminal Market. A bit of a different style, in that they don't scramble the meat, and it seems to get love-it or hate-it reviews, but if you're at the Market or the Pennsylvania Convention Center, and can't get to one of the others above, it's good enough.

A special note about the two famous places, Pat's and Geno's, 9th & Passyunk (pronounced pass-a-yunk). While they have their charms, the quality of the food isn't one of them. The atmosphere is certainly interesting, but if you go to either of these, just be aware that while you are getting a serviceable cheesesteak, you are not getting a good cheesesteak. Geno's is the least worst of the two, but Pat's is more famous.

One more note: while these places have menus, you do not have time to read them. When they ask what you want, just reply, "a wiz wit". Unless you are exceptionally fastidious, you may also want to consider standing while you eat rather than sitting. Most places have ample standing room. Consider yourself warned.