The first objective aimed to a deeper understanding of the Woodward-Hoffmann selection rules for pericyclic reactions to the development of new computational tools capable of treating reactivity problems in small to medium size (few tens of atoms) organic molecules. The second objective involved the development of protocols for mapping excited state energy surfaces and a systematic characterization of the photochemical reactivity of different classes of organic and biological chromophores. The corresponding results provided the first "statistical" demonstration of the ubiquity of conical intersectionsand singlet/triplet crossingsin organic chromophores and of their role, equivalent to that of transition states in thermal chemistry, as basic mechanistic elements in photochemistry.