While "life" may universally be a self-sustaining chemical system capable of Darwinian evolution, alien
life may be quite different in its chemistry from the terran life that we know here on Earth. In this case, it
will be difficult to recognize, especially if it has not advanced beyond the single cell life forms that have
dominated much of the terran biosphere. This review summarizes what we might infer from general
physical and chemical law about how such "weird" life might be structured, what solvents other than water
it might inhabit, what genetic molecules it might contain, and what metabolism it might exploit.