"The second step is to determine constitution, Le. which atoms are bonded to which and by what types of bond. The result is ex- pressed by a planar graph (or the corresponding connectivity mat- rix) ---- In constitutional formulae, the atoms are represented by letters and the bonds by lines. They describe the topology of the molecule." VLADIMIR PRELOG, Nobel Lecture, December l2;h 1975. In the present notes we describe the topological approach to the che- mistry of conjugated molecules using graph-theoretical concepts. Con- jugatedstructures may be conveniently studied using planar and...