I present a model for the stress required to extend or stretch continental lithosphere. The lithosphere is considered to have four rheological fields: quartz in the brittle field, quartz in the ductile field, olivine in the brittle field, and olivine in the ductile field. As continental lithosphere extends, the required stress initially decreases due to nearly isothermal thinning of the strong upper mantle (olivine near the brittle-ductile transition). The stress required to continue extending the crust may then increase due to cooling of the upper mantle. However, the required stress can increase to no more than about 1.3 times the stress required to start extension. This contrasts with an earlier study that required exponentially increasing stress to continue extension. My model improves on this earlier study by considering the effects of brittle failure in the upper mantle. The rheology of the continental lithosphere should be no barrier to its continued extension regardless of the rate of extension.