What are G-quadruplexes? In cells, nucleic acids are not simple linear polymers but adopt complex architectures – the classic Watson-Crick duplex and a variety of unusual folds. A fascinating structure is the G-quadruplex (G4). G-quadruplexes are a family of peculiar three-dimensional structural arrangements adopted by G-rich DNA and RNA strands, resulting from the stacking of several G-quartets, and stabilized by monocations such as sodium and potassium. The self-assembly of guanine derivatives has been known for more than a century, and the formation of G-quartets was proposed in the 1960s. Research into the relevance of G-quadruplexes has maintained an important momentum because these structures are stable under physiological conditions and there is increasing evidence pointing to biological roles for these conformations. The community of researchers interested in quadruplexes was initially comparable to the "fellowship of the Quartet", but has grown exponentially. A remarkable quantity of information has been obtained since the discovery of the G-quadruplex motif, which has made this field to grow from basic science to clinical applications. The pace of progress took another leap during the last decade, and quadruplexes changed status from lab curiosity to hot topic within a few years thanks to seminal contributions from several laboratories.