G-quadruplexes are unique structures formed by guanine-rich nucleic acid sequences, where four guanine bases form a square planar structure, stabilized by Hoogsteen hydrogen bonding. These structures can form in DNA, RNA, or hybrid sequences, and have been implicated in a variety of biological processes including DNA replication, gene regulation, and telomere maintenance. Research in this area focuses on understanding the structural features, formation, stability, and biological functions of these G-quadruplex structures. Interest in this field has grown in recent years due to the potential for targeting G-quadruplexes in cancer therapy and other diseases.