Do line baking pans with aluminum foil. This will make cleanup of anything from brownies to oil-drizzled roasted vegetables a snap. Food can stick to it, however, so you may want to coat regular foil with vegetable cooking spray or use a new sheet with each batch. Cookies will spread out a bit more, brown more on the bottom, and come out a bit crisper when baked on foil than on parchment, says Reynolds Kitchens tester Pat Schweitzer, so use foil for things like Belgian lace cookies and parchment for more substantial varieties.
Don't use foil to line the bottom of the oven to catch spills and drips. It will cause food to heat unevenly and, over time, can damage the oven, says Mary Ellen Camire, Ph.D., a product-development expert for the nonprofit Institute of Food Technologists. You're better off wiping up messes after each use, when the oven has cooled but before the spill has hardened.
Do use heavy-duty foil as a protective shield for poultry. To keep a chicken or turkey breast juicy and prevent the skin from burning before the rest of the bird has cooked (dark meat, which is higher in fat, cooks more slowly than white), cover it loosely with foil for the first couple of hours of cooking.
Don't use foil in a microwave oven. The electromagnetic waves pass through glass, paper, and ceramic, which don't overheat in the cooking process (they often feel hot after zapping because the food is still cooking inside). But aluminum deflects the waves, causing food to cook unevenly and possibly damaging the oven.
Do use foil to contain the odors of smelly foods before they go into the refrigerator or freezer. While high-quality plastic wrap works well, says Harold McGee, a food scientist and the author of On Food and Cooking: The Science and Lore of the Kitchen (Scribner, $40, amazon.com), foil wrapped with an airtight seal (press down hard on the creases) is even better because it's basically impermeable to odors and moisture. That airtightness means food well wrapped in foil is less prone to freezer burn, says Camire.
Don't use foil to store foods that are high in acids. This means tart fruits and dishes made with vinegar, tomatoes, or tomato sauce. After a few days in foil, the acids in lasagna, for example, interact with the aluminum and erode the foil, says McGee. Small amounts of aluminum can then migrate into the food, creating both pinprick holes in the wrap and a metallic taste in the lasagna. Also, white spots (actually aluminum salts) can form on these foods when their acidity reacts with the aluminum. Theoretically you can cut these spots away; they're not harmful. But they are certainly not appetizing either, so stick to plastic storage for the acidic goods.