The discovery of thousands of new planetary candidates over the last decade has revolutionized the field of planet formation. One of Kepler’s key findings is that the most abundant planets in our galaxy, observed to date, are larger than Earth but smaller than Neptune. Intriguingly, further observations have revealed a ‘radius valley' in the distribution of such small exoplanets. I will review the origin and formation of this small, close-in exoplanet population, demonstrating that the two planet populations located above and below the radius valley likely started out as one and that the smaller and closer planets lost their primordial hydrogen dominated atmospheres (super-Earths), while the larger and further away planets retained a significant fraction of their primordial envelope (sub-Neptunes). Comparing theory and observations, I will show that we can already infer important properties about the underlying planet population and I will discuss observational predictions and tests that can distinguish between photo-evaporation and core-powered mass loss models. I will conclude by sharing recent equilibrium chemistry results that link a planet’s metal-core, silicate-mantle, and hydrogen-rich atmosphere and discuss their implications for the total hydrogen mass budgets of sub-Neptunes, the formation and interior composition of super-Earths and observational tests.