In this article the research work carried out during the last two decades on the excited state behaviour of diarylolefins has been reviewed with the aim of investigating which are the structural factors responsible for the change of the photoreaction mechanism from the diabatic to the less common adiabatic photoisomerization, which leads to the product in the excited state. After a first part, where generalizations on the techniques, methodologies and kinetic treatments used for the study of adiabatic photoisomerization are described, a review of the main results available for diarylethenes, diarylbutadienes and distyrylbenzenes is presented, with emphasis on adiabatic singlet pathways, particularly studied in the authors` laboratory. The effects of the configurational and conformational structure and the medium polarity on the shape of the potential energy surfaces of the lowest singlet and triplet excited states, together with a specific investigation on the role of conformational equilibria on the photobehaviour, allowed a general interpretation of  the adiabatic mechanism to be proposed and some open questions to be underlined. The results of complementary fluorimetric, flash photolytic and photochemical measurements, often combined with theoretical calculations and with application of the statistical principal component analysis, are described with details on some representative cases.