This Reprint is dedicated to the memory of Enrique Pedroso, Professor Emeritus of Organic Chemistry at University of Barcelona, who passed away at the age of 72 on September 2020. Professor Enrique Pedroso has been one of the pioneers of Nucleic Acids Chemistry in Spain, significantly contributing to the development of this highly interdisciplinary field which combines organic chemistry, biochemistry, pharmacology, materials chemistry, and biophysics. His major research achievements were accomplished in the synthesis of modified oligonucleotides and especially conjugates and cyclic oligonucleotides, as well as their analogues, which opened up new avenues in the search for novel applications of oligonucleotides. This Reprint comprises a collection of research or review articles prepared by international experts in Nucleic Acids, including nucleoside and oligonucleotide synthesis, nucleic acids structural studies, DNA repair, and biophysical characterization of DNA-targeting ligands, especially G-quadruplex binding drugs.