Sense‐and‐avoid systems assumed increasing relevance in the last decade due to the ever‐increasing use of unmanned aerial vehicles (UAVs) for multiple applications. This implies the need of ensuring suitable level of safety of these platforms, where no pilot is onboard; therefore, first of all, the vehicle should be provided with the capability of seeing and being seen and also the capability of avoiding loss of separation and collision with respect to surrounding traffic. Many efforts have been made worldwide in order to study these aspects and to develop suitable technological solutions, taking into account several constraints that sense‐and‐avoid systems for UAVs have to comply with. Main components of these systems are the sense function, aimed to perform detection and tracking of the surrounding traffic, and the avoid function, aimed to provide early identification of possible conflicts (i.e., loss of separation and collision threats) and to elaborate suitable maneuvers to be performed by the UAV in order to resolve the identified conflicts. In this chapter, an extensive review is provided on the state of the art of the sense‐and‐avoid systems and methods, starting from the consideration of regulatory aspects, which are relevant for the technology development, then illustrating the main technologies available to perform the sense function, and, finally, describing the typical architectures of the sense‐and‐avoid systems proposed in literature and in the main related international projects.