Over the past years, three-dimensional (3-D) terrain mapping technology has improved to the degree that it is now extremely useful for site surveillance applications. Resolution and accuracy in absolute (world) coordinates of 1 m or better are now available. City-size areas can be collected and high-quality maps produced in a few days at reasonable cost. Maps are already available for many sites of interest and availability will increase as costs continue to drop and more applications are developed for them. The 3-D maps are useful in all phases of site security. I show how the maps are useful for planning, where they allow easy delineation of the areas to be monitored and optimum sensor placements. I show how the maps can be used for target detection algorithms, where the portions of each sensor's field of view that fall outside the area to be monitored can be masked out to reduce false alarms. Also, since the range to the map is known for each pixel within the sensor field of view, the scale of any potential target is also known and algorithms do not have to accommodate a wide range of potential target sizes. Finally, I show how electro-optical/infrared imagery can be projected onto the 3-D map to provide context. Previous detections, target tracks, and other information can also be added to the display to enhance its value. I have worked with a map of the Adelphi site of the U.S. Army Research Laboratory, projecting electro-optical and infrared imagery onto it with very encouraging results. I have also calculated sightlines for a radar considered for the roof of our main building. The tools are practical with current hardware at reasonable prices.