The discovery of exoplanets has brought the settings of many science fiction stories within reach of scientific inquiry. Astronomers’ ever increasing sensitivity to smaller and smaller planets has opened the opportunity for empirical insights into the nature and demographics of distant terrestrial worlds. Up to what size and mass do planets typically have rocky compositions? How Earth-like are these distant rocky worlds? How common are rocky planets in the Habitable Zones of their host stars? In this talk, I will present the current constraints on each of these questions, appealing both to individual planet case studies and to planet population statistics.