Meringue is optional. You can use a graham cracker crust, but the real thing is made with a pastry crust.
3 eggs, separated (separate when cold but use at room temp)
3/4 c key lime juice (6-7 limes)
1 can condensed milk
1/4 t cream of tartar
9 T sugar
8"-9" baked pastry shell
Beat yolks. Add condensed milk, beating well.
Add lime juice slowly and in small amounts. Otherwise, you'll curdle the custard. Beat well after every addition. Don't be in a hurry with this step.
Pour into baked pastry shell.
Beat egg whites (room temp and in a scrupulously-clean bowl with scrupulously-clean beaters and not even the littlest speck of egg yolk in them) until frothy and add cream of tartar. Increase speed of mixer and continue beating until soft peaks form.
Add sugar, one T at a time, beating to stiff peaks. When you are finished, not only should you have stiff peaks, but if you take a dab of meringue and "rub" it between your thumb and forefinger, you should not feel any "grit." If you do, keep beating because the sugar has not fully dissolved. Un-dissolved sugar makes the meringue "chewy." Don't be in a hurry with this step, either.
Spread meringue on pie, going all the way to the edge of the pie crust, so the meringue won't pull away from the crust. Mound extra meringue in center of pie.
Bake at 425 for about 4 1/2 minutes to brown meringue. (The custard filling has been "cooked" by the key lime juice.)
Cool to room temperature before serving.
To some people, a key lime pie is green, however, the real thing is yellow. I urge you to leave it out. If you must, start with 1 drop at a time.
Note: Here is a recipe for a never-fail pie crust. Easy to make. It's in this file about an appetizer/first course cheese pastry. Read down a couple of paragraphs.