Refrigerator, freezer, microwave & top-rack dishwasher safe Write + Erase lids for easy labeling of contents This Snapware set is made of Pyrex tempered glass, which is safe to use in both the microwave and oven. The lids are airtight to keep food fresher longer, yet are easy to put on and take off. Plus, you can write on the lids to eliminate the guesswork of what's inside. Then, erase the lid and reuse.