A monocular camera-based sense and avoid (S &A) system can possibly be the last chance to avoid nearby intruders or ground obstacles during an unmanned aerial vehicle (UAV) mission if the other systems cannot detect it. This article targets to summarize the main results of decade long S &A research in Institute for Computer Science and Control (SZTAKI) Hungary. After giving motivation and a historical review of SZTAKI S &A research it will first overview the object parameters measurable by a monocular camera. Detectability of different kinds of intruder vehicles and their avoidability will be discussed after. Then it discusses single camera system extension to a multi-camera one. The projection model of real world objects is the next topic as textbook projection models cannot be successfully applied so disc and rectangle projection models are proposed. The first approach of S &A decision is to assume linear, constant velocity own and aerial intruder trajectories, define time-to-closest-point-of-approach (TTCPA) and closest-point-of-approach (CPA) and estimate their values based-on the camera measurements. This is discussed in 2D (horizontal plane) and then in 3D. In the next step the extension of the method to steady and moving obstacles and non-straight, variable speed own flight trajectories is presented targeting to estimate also their size and global position. The first approach is compared to the new one giving application guidelines. A crucial issue is the decision about the type of the own flight trajectory for which a trajectory measure is introduced. After the summary of achieved results the future challenges are discussed.