This year marks the 20th anniversary of the first exoplanet discovered to orbit another star like our Sun. The field of exoplanets has since progressed dramatically, with a large part due to the success of NASA’s Kepler Mission, which has discovered and confirmed over 1,000 planets in our galaxy to date. Kepler’s primary goal is to determine the frequency of Earth-sized planets in the habitable zones (HZ) of other stars, regions within which a planet could sustain liquid water on its surface. In April 2014, we announced the discovery of Kepler-186f, the first Earth-sized planet found to orbit in the HZ of a star other than our Sun. While Kepler-186f is comparable to Earth in terms of size and the amount of starlight it receives, it orbits an M dwarf, a star that is smaller and cooler than the Sun. I will present our work on the discovery and confirmation of Kepler-186f and will present new models on how these planets may have formed. M dwarfs present a new set of challenges for planetary habitability, as planets orbiting in their HZs are subject to stellar flares, high speed impacts, and they orbit close enough to their star that they may become tidally locked. I will discuss our current knowledge on the potential habitability of planets around M dwarfs, and will compare their challenges to those faced by other HZ planets that have since been discovered around other stars that are more like our Sun. More than 70% of the stars in our galaxy are M dwarfs and therefore finding and characterizing Earth-size planets orbiting in their HZ has big implications for determining the frequency of other Earths.