  The Habitable Zone for a given star describes the range of circumstellar
distances from the star within which a planet could have liquid water on its
surface, which depends upon the stellar properties. Here we describe the
development of the Habitable Zone concept, its application to our own Solar
System, and its subsequent application to exoplanetary systems. We further
apply this to planets in extreme eccentric orbits and show how they may still
retain lifebearing properties depending upon the percentage of the total orbit
which is spent within the Habitable Zone.
