A flour made from specially treated ground corn, masa harina is the foundation for tortillas, tamales, sopes, and many other corn-based Mexican treats. To make masa harina, corn kernels are dried, then rehydrated and treated with lime (calcium oxide), which makes it possible to remove the skins. Once the skins are rubbed off, the kernels are thoroughly washed and ground into soft, pliable masa (dough). The fresh masa is then dried and powdered, becoming masa harina (harina means flour). Though fresh masa is generally preferable to masa harina, it's very perishable and therefore difficult to find outside of Mexico and its U.S. border states. Masa harina, on the other hand, has a shelf life closer to that of regular wheat flour. It's a pantry staple even in areas where fresh masa is available. To use it, you simply add water.
Look for masa harina at Latin markets and in the Latin section of well-stocked supermarkets. Maseca and Quaker are the most widely available brands.
Store masa harina as you would all-purpose flour: well-wrapped, in a dry place at room temperature for up to a year.