The objective of this project is to explore how a systems approach can be used to design and analyze systems for detecting nuclear material at our nation?s ports. This research uses discrete optimization and decision analysis models to design multi-layered, risk-based, port security systems for detecting nuclear weapons and materials. This project will develop discrete optimization to investigate risk-based port and maritime security systems; a decision analysis model to perform nuclear threat risk assessments on vessels; algorithms and heuristics to empirically assess and evaluate these models through a computational study and expert knowledge; explores the relationships between such models in order to provide multi-layered, system level port and maritime security that simultaneously detect nuclear material and cost-effectively deliver minimally invasive, high throughput screening systems using existing and future technologies and sensor networks. The broader impacts of the proposed research include the development of next-generation port and maritime security system paradigms that will push forward the boundary of achievable security at our nation?s ports, given existing and future screening technologies.