Some Styrofoam containers are microwavable. To determine which ones are microwave safe, check the packaging for labels announcing that the package is usable in the microwave.
Styrofoam is created from polystyrene, a form of plastic. While some plastics can leak toxins into foods, especially fatty foods and cheeses, the FDA tests plastic packaging to determine which ones are safe for use in the microwave. In its testing, the FDA considers the ratio of plastic surface area to food, the length of time the package is likely to be microwaved, how hot any food in the package may become and how often someone is likely to eat from the package.