There are two main varieties of this small green citrus fruit: the Persian lime and the Key lime. The most common (and widely-available) is the Persian lime which is about the size of a small lemon with thin green skin. The Key lime, which is much rarer, is smaller, more acidic, and has a strong, citrusy aroma. Key limes have thinner skin, which tends to be yellow-green, or completely yellow when ripe.
1 medium lime (about 4 oz.) = 3 Tbs. unstrained juice; 4 tsp. lightly packed zest (when grated with a rasp) or 2 tsp. zest (when grated on a box grater).
Look for limes that seem heavy for their size, promising more juice. Also keep an eye out for plump, glossy skin—indications that the rind will be rich in flavorful oils. Choose Persian limes that are about 2 inches in diameter, fragrant, and plump, ith smooth, medium-green skin. Choose Key limes that are about 1-1/2 inches in diameter with smooth greenish-yellow or yellow skin, and a strong lime aroma.
Our favorite zesting tool is a rasp-style grater which easily turns the rind into a pile of tiny, feather-light bits without digging into the bitter white pith beneath.
For juicing, a plastic or wooden reamer is a fine choice, but a citrus press is even more efficient if you're juicing a lot of limes. If you roll the lime on the counter with medium pressure before cutting and juicing, it will yield more juice.
If you use them often, you can store limes at room temperature. For longer storage, keep them somewhere cool and dry, preferably in a basket or net bag to allow for air circulation, which prevents mold. If you store limes in the refrigerator, don't crowd them.