Since potential therapeutic significance had been discovered for quadruplex secondary structures of nucleic acids, many compounds stabilizing these targets were identified. The progress was due to understanding the structural features of G-quadruplexes. Quadruplex ligands selectively suppress the growth of tumor cells by indirectly attenuating telomerase activity or inhibiting oncogene expression. A therapeutic effect demonstrated in vivo supports the idea that G-quadruplexes are promising for the development of targeted anticancer drugs. The review highlights the significance of G-quadruplexes as therapeutic targets and focuses on the biochemical properties of low-molecular-weight quadruplex ligands.