The prototypic and best studied example of biomolecular evolution 
is that of RNA molecules.  Their replication and mutation, mimicking 
processes in nature, can be studied in the test tube and by computer
simulation. RNA evolution can be understood in quantitative terms on 
the basis of a mapping from an RNA sequence space into a space of 
RNA structures.  The RNA sequence-structure map and its properties 
will be  discussed in terms of different physical approaches supported 
by theoretical formulation.