Guanine (G)-rich sequences can form non-canonical four stranded structures known as G-quadruplexes (G4s). The G4 structure has been found in biologically significant regions of the genomic DNA, and many G4 ligands with a high specificity and affinity have been reported. In this review, we focus on the functional G4 ligands, which not only bind to the G4 structure but also have some function, especially with inducing G4 folding, chemical reactions, and alkylation by proximity.