Most planets are thought to form through the 'core accretion' process. This process can be probed by comparing the occurrence rates of extrasolar planets of different masses and orbital distances. Until recently, the evidence was limited to massive, Jovian planets. This talk will focus on recent results that probe much smaller planets: 1) the Eta-Earth Survey, a radial velocity planet search that provides a census of nearby extrasolar planets with masses of 3-1000 Earth-masses; and 2) the Kepler mission, which detects transiting planets with sizes down to one Earth radius. We are learning that close-in, 'super-Earth'-size planets are ubiquitous and that the models of planet formation and migration need substantial revision to account for them.