In this chapter, we introduce the use of several unmanned aerial vehicles (UAV) for 3D UAV positioning and sensing. In particular, we investigate a problem of using one active unmanned aerial vehicle (UAV) and four passive UAVs to localize a 3D target UAV in real time. In the considered model, each passive UAV receives reflection signals from the target UAV, which are initially transmitted by the active UAV. The received reflection signals allow each passive UAV to estimate the signal transmission distance which will be transmitted to a base station (BS) for the estimation of the position of the target UAV.