Security is a critical concern around the world, whether it is the challenge of protecting ports, airports and other critical infrastructure, interdicting the illegal flow of drugs, weapons and money, protecting endangered species, forests and fisheries, suppressing urban crime or security in cyberspace. Unfortunately, limited security resources prevent full security coverage at all times; instead, we must optimize the use of limited security resources. To that end, we founded the "security games" framework to build decision-aids for security agencies. Security games is a novel area of research that is based on computational and behavioral game theory, while also incorporating elements of AI planning under uncertainty and machine learning. We have deployed securitygames based decision aids for infrastructure security such as at the ports and ferry traffic with the US coast guard (in the ports of New York, Boston, Los Angeles/Long Beach, Houston and others), for security of airports and air traffic with the US Federal Air Marshals and the Los Angeles World Airport (LAX) police, and tested this framework for security of metro trains with the Los Angeles Sheriff's Department. Moreover, recent work on "green security games" has led to testing our decision aids for protection of fisheries with the US Coast Guard and protection of wildlife at sites in multiple countries, and opportunistic crime security games have focused on suppressing urban crime. This chapter will discuss applications of security games, and outline research challenges in security games including algorithms for scaling up security games as well as for handling significant adversarial uncertainty and learning models of human adversary behaviors.