Not all glass is designed to be used in the microwave, so stick to products labeled "microwave safe." If the glassware isn't labeled, check by microwaving the container on high for a minute or two and give it a quick feel. If the container is hot, it's not safe to use for cooking in the microwave. A slightly warm or cool glass is microwave safe.
Video of the Day
Concerns With Glass
Glass that is not microwave safe may have tiny air bubbles that could expand during heating. If these bubbles expand enough, the glass will shatter. Glass trimmed with metal is not microwave safe. The metal could spark and cause the glass to shatter. If your glass is colored, look for a "microwave-safe" label before using it in the microwave. The dyes used to color the glass may not be food safe.
Ceramic, which is fired clay, is not technically glass but is also microwave safe if labeled as such. Some glass-ceramic dinnerware may shatter if heated in the microwave and then cooled too quickly; it will not be labeled as "microwave safe." Metal, including foil and metallic take-out containers, brown paper bags, dairy storage containers and foam-insulated cups and trays are not designed for use in the microwave. Some plastics are microwave safe, while others are not. You're best off not using plastic, including plastic wrap, unless it's labeled "microwave safe."