Abstract: Sub-Neptune and super-Earth-size planets are a new category of astrophysical objects. Though absent from the Solar System, exoplanet surveys have revealed that they are a dominant outcome of planet formation found in abundance around other stars. The nature of these planets is not well understood. In the sub-Neptune size range a large variety of planet bulk compositions are a priori possible, including terrestrial super-Earths, mini-Neptunes with hydrogen-helium envelopes, and water-worlds with several tens of percent water by mass. In this talk, I will present recent results from my group aimed at understanding the nature and origin of these enigmatic sub-Neptune-size worlds and disentangling the relative contributions from various compositional scenarios to the observed population of planets.