Reactive systems are software systems that engage in a continual dialogue with their environment. They constitute the software parts of cyber-physical systems where timely reactions are often critical to safety. Applications include autonomous vehicles, electric power systems, industrial automation, healthcare electronics, and robotics. Because the software engages in a continual dialog with its environment, it often has conflicting requirements. It needs to be predictable, but robust to unpredictable events; it needs to react in a timely manner, but this often requires reacting with inconsistent information; it needs to be adaptable, but demonstrably safe; and it needs to be secure, but accessible and available. Many conferences and workshops focus on one of the goals, such as achieving real-time behavior, without explicitly acknowledging the costs and without providing sound strategies for dealing with failures that prevent reaching the goals. The focus of this seminar will be on the tradeoffs that are intrinsic in the design of such systems. When you make a system predictable, available, secure, or even demonstrably safe, what have you lost? This Dagstuhl Seminar will pull in experts from manifold disciplines to identify and discuss the fundamental limits in reactive systems design that make tradeoffs inevitable.