Super-Earth and sub-Neptune sized planets are a new category of astrophysical objects. Though absent from the Solar System, they are found by exoplanet microlensing, radial velocity, and transit surveys to be common around distant stars. The nature of planets in this regime is not known; terrestrial super-Earths, mini-Neptunes with hydrogen-helium gas layers, and water-worlds with several tens of percent water by mass are all a-priori plausible compositions. Disentangling the contributions from each of these scenarios to the population of observed planets is a critical missing link in our understanding of planet formation, evolution, and interior structure. I will review individual highlights from the diverse complement of sub-Neptune-size planets discovered to date, and present a statistical analysis constraining the fraction of planets that are rocky as a function of planet size. I will conclude by describing avenues forward toward identifying bulk composition trends in the growing census of known exoplanets, and connecting these composition trends back to distinct planet formation pathways.