This book provides current research on G-quadruplex structures, their formation and role in biology. Chapter One summarizes the structural features of quadruplexes built by sequence extensions and site-specifically incorporated modified building blocks that have been established to induce the formation of stable, majority single G-quadruplex folding topologies, primarily from the human telomere repeat DNAs. Chapter Two discusses the G-quadruplexes and i-motifs as scaffolds for molecular engineering of DNA aptamers. Chapter Three examines the case of the natural alkaloids coptisine and chelerythrine. Chapter Four describes the G-quadruplex induced fluorescent signals that have been employed for the study of their folding and unfolding mechanism and thereby visualisation of their formation and structure. Chapter Five reviews the historical aspect of how major biological roles of G-quadruplexes have been uncovered by the use of specifically designed small molecule probes; and discusses the future applications of G-quadruplex interacting small molecules, including the development as potential drug candidates. Chapter Six focuses on the latest advances in the development of the telomerase assays and the authors further discuss the application potentiality of the assays for clinical samples.