A key question regarding the origin of life is how Earth became a habitable planet and
why its neighbors, Venus and Mars, met a different fate. Addressing this question helped
lead to the development of the habitable zone (HZ), which is the region around a star
where stable bodies of water could exist on the surface of a rocky planet. In here, I
briefly review how the HZ is defined, including how it is tied to the habitability of
the 3 terrestrial planets.  Although the classical HZ has been traditionally defined for
the main-sequence phase of stellar evolution, I discuss why the pre-main-sequence HZ is
crucial for distinguishing worlds with Earth-like atmospheres from those with dense CO2
(e.g. Venus) or tenuous CO2 (e.g. Mars) atmospheres. Planetary accretion models also
predict a fourth intriguing possibility: water worlds with water contents that far
exceed Earth”Ēs. Using 1-D radiative-convective climate, atmospheric escape, and stellar
evolutionary models, I calculate the pre-main-sequence HZ for both solar and M-stars and
discuss likely atmospheric compositions for targets of interest, like TRAPPIST-1 and
Proxima Centauri b. I also compare my assessments against those predicted by current
planetary accretion models.