Guanine quadruplexes (GQs) are four‐stranded DNA/RNA structures exhibiting an important polymorphism. During the past two decades, their study by time‐resolved spectroscopy, from femtoseconds to milliseconds, associated to computational methods, shed light on the primary processes occurring when they absorb UV radiation. Quite recently, their utilization in label‐free and dye‐free biosensors was explored by a few groups. In view of such developments, this review discusses the outcomes of the fundamental studies that could contribute to the design of future optoelectronic biosensors using fluorescence or charge carriers stemming directly from GQs, without mediation of other molecules, as it is the currently the case. It explains how the excited state relaxation influences both the fluorescence intensity and the efficiency of low‐energy photoionization, occurring via a complex mechanism. The corresponding quantum yields, determined with excitation at 266/267 nm, fall in the range of (3.0–9.5) × 10 −4 and (3.2–9.2) × 10 −3 , respectively. These values, significantly higher than the corresponding values found for duplexes, depend strongly on certain structural factors (molecularity, metal cations, peripheral bases, number of tetrads …) which intervene in the relaxation process. Accordingly, these features can be tuned to optimize the desired signal.