NASA's Kepler Mission has revealed that the most common size of planet in our galaxy may be those from 2-3 Earth radii. Such medium-sized planets are significantly more common on close-in orbits than Neptune and Jupiter-class giant planets. We have no analog for these planets in our solar system. What are they made of? An example relatively close to home is planet GJ 1214b, which is 2.6 Earth radii and 6 Earth masses, and orbits an M star near the Sun. This planet has been extensively studied with the Hubble and Spitzer Space Telescopes. In this talk I will discuss our current understanding of the composition and atmospheric physics of GJ 1214b, which is potentially the prototype for this class of low-mass low-density planets.