The extremities of chromosomes end at telomeres in a G-rich single stranded overhang that may adopt peculiar structures such as T-loop and G-quadruplex. G-quadruplex is a poor substrate for telomerase activity and different classes of small molecule ligands that selectively stabilize this structure and inhibit telomerase activity have been selected by screening or synthesized by oriented chemistry. These ligands differ from catalytic inhibitors of telomerase by several points that were discussed in the present review, with a special emphasis on their biological activity as potential antitumor agents.