The remarkable exoplanet discoveries made by the Kepler and K2 missions have enabled astronomers to begin to piece together the history of the Earth and to understand how and why it differs from its diverse exoplanetary cousins. Two still outstanding puzzles include the differences between the formation and evolution of rocky versus non-rocky small planets, and why there seem to be a size gap with very few exoplanets at or about two Earth-radii in size (planets with smaller radii are likely to be rocky or Earth-like in their composition). In order to estimate an exoplanet’s composition its density is needed, requiring a measurement of mass as well as size. While a radius can be estimated from the shape of the planet’s transit curve as it blocks out its host star’s light, a mass is more difficult to determine. In order to develop the emerging picture, however, precise and accurate masses are required for more planets that are similar in size to the Earth.