Aluminum foil is useful for wrapping, covering, and catching drips, but several familiar rules are myths.
The safest method depends on the type of foil, the appliance, and the food touching it.
With standard household aluminum foil, the shiny and dull finishes come from manufacturing. Either side can touch food, and the difference does not meaningfully change cooking.
Non-stick foil is the exception. Its coated side should face the food, following the directions printed on the package.
Foil on the bottom of an oven can trap heat, interfere with airflow, or damage the surface. Use a rimmed tray or a limited sheet on the rack below the food instead.
Metal can cause arcing in a microwave. The USDA permits only limited shielding in certain situations, but the appliance manual takes priority. Never completely wrap food in foil for microwaving, and stop immediately if sparks appear.
Tomato, citrus, salty, or highly seasoned foods may cause pinholes or discoloration after prolonged contact. The USDA describes the resulting aluminum salt as harmless, but glass or a suitable reusable container avoids the reaction and preserves appearance.
For ordinary foil, shiny side or dull side makes no practical difference. The important rules are to avoid lining the oven floor, treat microwave use cautiously, and choose another container for prolonged contact with acidic or salty food.