This chapter describes the processes of base/base dimerization that UV absorption induce in nucleic acids. In the absence of any sensitizer dimerization principally involves the singlet excited electronic states. Focussing on bipyrimidine dimers, which are those produced with the largest yield, we describe which are the major photodimers formed for each sequence, the underlying basic molecular mechanisms and the main experimental features associated to these reactions. We show that different electronic states are associated to different photoproducts and we highlight the importance of the local effects in determining the yield found for the different sequences. In particular, the structure adopted by the reactive dinucleotide step when absorbs UV light plays a major role.