That is the title of the famous Schrödinger's book (1944), which inspired to scientists as Crick. Now, after the great discoveries in Biology, mainly in Molecular Genetics, the question remains. We know essential aspects of living organisms: need of some type of informative auto-replicating molecule and an isolated medium where biochemical reactions occur in a stationary state far from equilibrium. But the great diversity of life forms in our planet, exemplified by extremophylic organisms, make the question yet non-trivial: What are the minimal conditions inherent to any real or potential living structure? For example, is Carbon an essential atom to life? Or individuality of systems with membranes? And the old question on virus: are they living forms? Or a transposon? And a prion?