Guanine-rich nucleic acid sequences are known to form G-quadruplex - four-stranded DNA or RNA structures stabilized by an array of Hoogsteen hydrogen bonds. G-quadruplex structures are involved in the modulation of gene expression at the transcription and translation levels. Accordingly, G-quadruplexes are considered as novel therapeutic targets for anticancer drug development. In this review, the authors provide a brief, up-to-date summary of G-quadruplex binding ligands, including naturally occurring molecules, synthetic compounds, and molecules identified by computational database screening. The key structural motifs of G-quadruplex binding ligands, that is, an aromatic core and basic side chains, are addressed in the context of how these molecules interact with G-quadruplex. A better understanding of these interactions would facilitate the rational design of ligands selective for DNA or RNA G-quadruplex.