Pyrex is safe to use in the oven. Pyrex is a heat-tempered glass designed to withstand changes in temperature. The plastic lids of Pyrex containers and casserole dishes are not oven safe.
Originally, Pyrex glass was designed to be used in railroad lanterns. The use of Pyrex as ovenware was accidental and borne out of one woman's immediate need for a casserole dish. She used a portion of broken battery jar to replace her dish, which resulted in the beginning of a new type of heat-resistant cookware that is still in use today. Today, Pyrex sells many different kinds of ovenware and kitchen products, including casserole dishes, food storage containers and measuring cups.