  Here, we evaluate our nearest planetary neighbor, Venus, as an exemplar of
the runaway greenhouse state that bounds the inner edge of the habitable zone.
Despite its current hellish surface environment, Venus may once have been
habitable with oceans of surface liquid water. Over time, it lost its
potentially clement environment as the sun brightened. Today, it represents the
end-state of habitable planet evolution, and it therefore provides valuable
lessons on habitability as a planetary process. Beyond the solar system,
exo-Venus analogs are likely common types of planets, and we likely have
already discovered many of Venus' sisters orbiting other stars. Furthermore,
our near-future exoplanet detection and characterization methods are biased
towards observing them. Therefore, it is instructive to consider what Venus can
teach us about exo-Venus analogs. By observing exo-Venus planets of differing
ages in in different astrophysical contexts, these distant hot terrestrial
worlds may likewise allow us to witness processes that occurred on Venus in the
past.
