G-quadruplexes (G4) are non-canonical DNA secondary structures which can form in guanine-rich DNA sequences. G4s can be unimolecular, bimolecular or tetramolecular, and structures vary in relative strand direction, loop length and composition, and glycosidic bond rotation. Over 700,000 G4 sequences have been identified in the human genome using nextgeneration genome sequencing. Both experimental methods and computational algorithms suggest a high percentage of G4-forming sequences are found in gene promoter regions and telomeres. The formation of G4 structures in these regions is thought to inhibit the growth of cancer cells by inhibiting telomere maintenance and impeding the transcription of oncogenes. The use of G4-stabilising ligands is therefore being explored as a potential molecular therapeutic approach to the suppression and elimination of cancerous cells. As such, structural insight into G4s and G4-ligand complexes is useful to assist in ligand design.