Prior to introducing and describing these three gaps, the different border types (airport, green and blue) should be introduced. Airport borders are the least challenging for practitioners to control. Border crossers arrive in a confined space, are visible as they walk through various checkpoints, their papers are checked quickly against information in databases and they are observed for unusual and suspicious behaviours by a large number of border guards, dogs and their handlers, and occasional profilers. Land (Green) borders are similar to airport checks only for the established border Crossing points (BCPs). Most of the land border management challenges are encountered along the land borders between BCPs. The distances between the BCPs are the majority of the land borders. Security along this border type is composed of many distinct elements, including physical and artificial barriers, the deployment of border patrol personnel and installation of surveillance technological means like long-range radars, sensors sprinkled on suspected routes, patrols by vehicles and UAVs and observation towers. In general, border controls which years ago were handled by military units, have been replaced by technology, fast response units from Border Guard Authorities (BGAs) when alerted and the assistance of national police. Once suspected illegal crossers are detected, border guards can be dispatched to intercept them if possible, and local national police is notified of the incursions of unwanted and irregular crossers for further law enforcement actions within the internal space of the EU. Sea (Blue) borders present their own challenges. Their management requires massive investments in vessels and observation technology to detect small boats before they reach the territorial waters and shores of the EU.