Supramolecular organization and interchain interactions play a key role in the (photo)-physical and (photo)-chemical processes that take place in p-conjugated polymers.  They impact therefore the performance of polymer-based devices such as light-emitting diodes and photovoltaic cells.  Our goal in this lecture is: (i) to describe the modifications that supramolecular organization brings to the electronic structure of the polymer with respect to the situation for an isolated chain; (ii) to illustrate how such interactions can strongly quench luminescence and to propose strategies that can help in preventing luminescence quenching in the solid state; and (iii) to illustrate the cases where specific intermolecular interactions can be used to promote electron-transfer processes that can be exploited in photovoltaic cells.