M dwarf stars are especially attractive targets in the search for Earth-like planets because small planets are easier to detect around such stars, and the “habitable zone” is closer to the star where planets are more readily detected. The Kepler mission has discovered Earth-size planets transiting M dwarfs, including some that may orbit in the habitable zone. But the properties of these stars, and hence those of their planets, are poorly constrained. We have refined the temperatures of nearby M dwarfs whose angular diameters have been measured by interferometry. There is excellent agreement between certain model spectra and the observed spectra of these stars and we use “spectrothermometry” to estimate the temperatures of distant Kepler M dwarfs with an accuracy of 60K. Model-independent empirical relations are then applied to estimate stellar radius, luminosity, and mass, and use these in turn to revise the radii and stellar irradiances of M dwarf planets: a recent estimate of the occurrence of Earth-size planets in the habitable zones of M dwarfs may need to be revised downwards. The properties of these planets lead one to speculate on their internal structure, atmospheres, and habitability, which I shall do.