Over the last decades, the importance of UAVs in general aviation has grown tremendously. Although at first, UAVs were only used in wartime, the last years they have proven to be capable of playing an important role in civil applications as well. Up till now however, there are no clear regulations on how to integrate these UAVs into the airspace and make them coexist with manned aircraft. This book wants to help incorporating UAVs into controlled airspace by discussing the main anti- collision systems and the attached regulations. In a first part, a general discussion is held about UAVs, including amongst others a clear definition, a classification and an overview on their possible applications. After the description of existing UAV regulations, this book has above all the intention to highlight the safety issues that present themselves when joining manned and unmanned aircraft together and to offer possible solutions to counter one of these issues, namely mid-air collisions. Several sense and avoid applications are described and rated and finally a recommendation is made about the best solution possible. A brief word on the future concludes this essay.