The habitable zone (HZ) around a star is typically defined as the region where a rocky planet can maintain liquid water on its surface. That definition is appropriate, because this allows for the possibility that carbon-based, photosynthetic life exists on the planet in sufficient abundance to modify the planet’s atmosphere in a way that might be remotely detected. Exactly what conditions are needed, however, to maintain liquid water remains a topic for debate. In the past, modelers have restricted themselves to water-rich planets with CO₂ and H₂O as the only important greenhouse gases. More recently, some researchers have suggested broadening the definition to include arid, “Dune” planets on the inner edge and planets with captured H₂ atmospheres on the outer edge, thereby greatly increasing the HZ width. Such planets could exist, but we demonstrate that an inner edge limit of 0.59 AU or less is physically unrealistic. We further argue that conservative HZ definitions should be used for designing future space-based telescopes, but that optimistic definitions may be useful in interpreting the data from such missions. In terms of effective solar flux, S_eff, the recently recalculated HZ boundaries are: recent Venus—1.78; runaway greenhouse—1.04; moist greenhouse—1.01; maximum greenhouse—0.35; and early Mars—0.32. Based on a combination of different HZ definitions, the frequency of potentially Earth-like planets around late K and M stars observed by Kepler is in the range of 0.4–0.5. Significance National Aeronautics and Space Administration’s (NASA) Kepler Space Telescope has detected over 3,000 planet candidates, about a dozen of which are probably rocky planets within the liquid-water habitable zones of their parent stars. Climate-modeling calculations discussed here shed light on the width of that zone. Within the next several years, NASA may obtain spectra of nearby transiting Earth-sized planets around M stars, using its James Webb Space Telescope. NASA hopes to build an even more capable space telescope to perform direct imaging of Earth-like exoplanets and take spectra of their atmospheres. Once data are obtained from either of these missions, correct interpretation of possible biomarker gases will become critical. We discuss here how those interpretations might be made.