We show that extrasolar planets in orbit around nearby stars can be detected from the ground or from a stratospheric telescope in the infrared region of the spectrum. We present calculations on the detectability of extrasolar planets, using Erich Grossman's Atmospheric Transmission (AT) code to compute atmospheric transmission at bands centered at 11 μm, 20 μm, 27 μm, 200 μm, 225 μm, 350 μm, 450 μm, 640 μm, 750 μm, and 870 μm. Detection limits for terrestrial and Jovian planets orbiting Sun-like stars are presented by assuming Planck emission. We consider several potential sites including the south pole, a 5000-m elevation site in the Atacama Desert in Chile, Mauna Kea in Hawaii, and an aerostat-borne telescope flown near the poles. We consider extrasolar planet detection with a 10-m class mid-IR telescope and a dilute aperture stratospheric telescope consisting of 4-m mirrors optimized for this task. Detection in the submillimeter, even with a 104 m2 collecting area array is extremely difficult because of low atmospheric transparency, and the decrease (∝λ-2) in planet emission with increasing wavelength in the Rayleigh-Jeans limit. We discuss critical technologies needed for this undertaking, including tethered aerostats and balloon-borne telescopes, the development of mid-IR nulling interferometry, actively cooled optics operating in the atmosphere, and optimized filters that are matched to the atmospheric transmission windows. ¿ American Geophysical Union 1996