oncrete structures are the basis of contemporary technical civilization. They support transportation, storage of goods, and shelter humans in large numbers. Despite their endurance, they have ceased to be considered maintenance-free. They are subject to long-term loading due to their own weight, dynamic loading scenarios including earthquakes, as well as environmental degradation. Concrete structures should undergo inspection and evaluation of their health condition, and proper maintenance actions should be proposed. In order to prolong their useful lifespan, a robust monitoring methodology should be applied. The first and basic information is extracted from visual inspection. However, this concerns only the surface distress indications. Defects may start to grow from the interior, and therefore, an assessment based solely on visual observation would not be complete. Among monitoring methodologies that assess the full volume of the structure, acoustic emission (AE) has started to be applied on a wide basis. AE is a passive technique that offers specific advantages like the real-time monitoring of defect propagation, which enables the characterization of the critical moments of the structure in relation to the applied operational load. Three-dimensional localization of the cracking sources is also possible if a number of sensors are used, while the parameters of the recorded waveforms exhibit strong sensitivity to the mode of cracking, enabling estimations of the dominant fracture processes. Additionally, accessibility is limited to just a few points of the surface, through which large structural volumes are inspected, highlighting the global nature of the technique. In this chapter, the basics of AE are discussed in terms of instrumentation and correlation of AE parameters to damage of concrete structures, and current studies aimed at improving the characterization reliability are also discussed.