"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 connec­ ted graphs because they reflect in the simple way the connectivity of their pi-centers. Connectivity is important topological property of a molecule which allows a conceptual qualitative understanding, via a non­ numerical analysis, of many chemical phenomena or at least that part of phenomenon which depends on topology. This would not be possible sole­ ly by means of numerical (molecular orbital) analysis.