The article reviews the characteristics and functions of G-quadruplex (G4) structures, a type of non-B-form DNA secondary structure. G4 structures, formed by stacks of guanine molecules, are stable and can occur in both intramolecular and intermolecular forms. These structures are conserved in evolution and are found in various genomic regions, including telomeres, promoters, and replication origins. The review highlights the role of G4 structures in genome stability, transcription, and other cellular processes. It discusses the biochemical properties of G4 structures, their formation and resolution by helicases, and their potential pathological implications. The article also explores the involvement of G4 structures in epigenetic regulation, meiosis, and recombination. Despite the wealth of in vitro data, direct evidence for G4 structures in vivo is limited, and the field continues to seek methods to confirm their existence and functions.The article reviews the characteristics and functions of G-quadruplex (G4) structures, a type of non-B-form DNA secondary structure. G4 structures, formed by stacks of guanine molecules, are stable and can occur in both intramolecular and intermolecular forms. These structures are conserved in evolution and are found in various genomic regions, including telomeres, promoters, and replication origins. The review highlights the role of G4 structures in genome stability, transcription, and other cellular processes. It discusses the biochemical properties of G4 structures, their formation and resolution by helicases, and their potential pathological implications. The article also explores the involvement of G4 structures in epigenetic regulation, meiosis, and recombination. Despite the wealth of in vitro data, direct evidence for G4 structures in vivo is limited, and the field continues to seek methods to confirm their existence and functions.