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London Broil is a generic term that's applied to many cuts of meat. It doesn't specify an actual source for the cut, so it's a good idea to find out where on the animal your particular piece of meat came from. Oftentimes the label will mention this below the words London Broil, but not always. It's very helpful to know for sure, as the method for prepping one that's actually a relatively thin flank steak will be somewhat different from one that's just a big thick piece of top round.

That said, London Broil cuts are generally prone to being very tough, and should therefore be cooked quickly, at high heat (broil, grill, sear), to a final doneness of no more than medium-rare. Be sure to allow for carry-over cooking during resting, which should be about 15 minutes. Then, slice it very thinly, at a slight angle and across the grain before serving.

Hey Loco, do you recommend I marinate the steak first?

I agree with LoCo's advice. The key is not to overcook. Also try a marinade before hand.
My uncle used to marinate his London Broil with a mix of apple juice, worcestershire, and montreal steak seasoning. Not swimming in it, but enough to coat both sides, flip once during an overnight soak. Pat off the excess marinade before grilling to ensure a coat sear on the exterior.
My mom would do a sort of teriyaki treatment, but include a specific type of fruit in the marinade, such as a pureed/mashed kiwi, passionfruit, mango or piece of pineapple. This is ONLY advisable for quick marinades on tougher cuts because there is an enzyme in these fruits which break down the meat. BIG CAVEAT: This can render your meat into a mushy mess if you leave it too long (e.g. several hours to overnight). If you use these types of fruit in your marinade, marinate only for an hour, wipe off excess, set aside until ready to cook, but cook it soon.
Some times for fajitas, I will buy flank steak. Squeeze fresh lime juice over both sides, rub it in, then rub with olive oil, and then rub a mix of spices (ancho chilii, chipotle chili, salt, toasted cumin seed, cayenne, & black pepper--I grind it all together with my mortar & pestle) and throw it on the grill within the hour. Grill to rare or med rare at most. Tender, juicy and flavorful everytime.

I've always had great luck with the top round for london broil. Marinate it, cook it HOT and fast. Carve across the grain as thinly as possible.

Btw, I always play a game with myself...will not pay more than $1.99 lb for it, then stock up. Keep an eye out for specials.

While my favorite is filet mignon I'm certainly experimenting with other cuts because of the prices. I will be trying a London Broil soon myself, though need to remember what the cuts are out are local stores so I don't get too tough meat, so thanks for asking this question. I just bought some Delmonico's today to grill tomorrow night and am about to recipe hunt.

I am a huge fan of marinades, especially a balsamic vinegar/brown sugar glaze I use from Martha Stewart. If you can find flat iron steaks, I think those marinated for 6-8 hours in the balsamic and grilled properly taste exactly like filet mignon, and at 1/3-1/4 of the price it's one of my new favs.

Good luck with your meat!

Don't forget flank steak, it is so delicious. Takes a marinade very well, and has such a rich flavor. Just be sure to sear and slice thinly against the grain. Flank steak is what proper fajitas are made from.

My opinion is to ditch the London Broil and hunt down a Tri-tip. It's mostly a westcoast cut of meat, but once you've had it you will wonder why you can't get it in every grocery store or meat market.
I typically dry rub the Tri-tip with garlic salt, pepper or pepper blend, a little paprika, and sometimes curry.
If I marinade it, I do it with about 5 parts BBQ sauce and 1 part brown mustard. My wife prefers meat well done while I like it more on the rare side. Tri-tip is not a uniform thickness cut, so I can usually satisfy both of us and take it off the heat at one time in one piece. I grill it over medium/low heat for about 1 1/2 hours, keeping the fat side up most of the time. Keep it from searing too much. Delicious!!!